<PAGE>
 
     AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON JUNE 10, 1994
                                                      REGISTRATION NO. 33-53107
- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------
                      SECURITIES AND EXCHANGE COMMISSION
                            WASHINGTON, D.C. 20549
                                ---------------
                              AMENDMENT NO. 3 TO
                                   FORM S-4
                            REGISTRATION STATEMENT
                                     UNDER
                          THE SECURITIES ACT OF 1933
                                ---------------
                  UAL CORPORATION AND UNITED AIR LINES, INC.
          (EXACT NAME OF EACH REGISTRANT AS SPECIFIED IN ITS CHARTER)
                                ---------------
    DELAWARE--UAL                  4512--UAL            36-2675207--UAL
   DELAWARE--UNITED              4512--UNITED          36-2675206--UNITED
                         (PRIMARY STANDARD INDUSTRIAL   (I.R.S. EMPLOYER
   (STATE OR OTHER        CLASSIFICATION CODE NUMBER)IDENTIFICATION NUMBER)
   JURISDICTION OF
   INCORPORATION OR
    ORGANIZATION)
 
                           1200 EAST ALGONQUIN ROAD
                      ELK GROVE TOWNSHIP, ILLINOIS 60007
                                (708) 952-4000
      (ADDRESS, INCLUDING ZIP CODE, AND TELEPHONE NUMBER, INCLUDING AREA
              CODE, OF REGISTRANTS' PRINCIPAL EXECUTIVE OFFICES)
                                                     Copies to:
      LAWRENCE M. NAGIN, ESQ.                 PETER ALLAN ATKINS, ESQ.
          UAL CORPORATION                     THOMAS H. KENNEDY, ESQ.
           P.O. BOX 66100                      ERIC L. COCHRAN, ESQ.
      CHICAGO, ILLINOIS 60666             SKADDEN, ARPS, SLATE, MEAGHER &
           (708) 952-4000                               FLOM
   (NAME, ADDRESS, INCLUDING ZIP                  919 THIRD AVENUE
    CODE, AND TELEPHONE NUMBER,               NEW YORK, NEW YORK 10022
 INCLUDING AREA CODE, OF AGENT FOR
              SERVICE)
                                ---------------
  APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE TO THE PUBLIC: At the
effective time of the recapitalization (the "Recapitalization") of UAL
Corporation described in the Proxy Statement/Joint Prospectus forming a part
of this Registration Statement.
 
  If the securities being registered on this form are being offered in
connection with the formation of a holding company and there is compliance
with General Instruction G, check the following box. [_]
                                ---------------
                        CALCULATION OF REGISTRATION FEE
- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------

<TABLE>
<CAPTION>
                                               PROPOSED       PROPOSED
 TITLE OF EACH CLASS OF        AMOUNT          MAXIMUM        MAXIMUM
    SECURITIES TO BE            TO BE          OFFERING      AGGREGATE        AMOUNT OF
       REGISTERED            REGISTERED     PRICE PER UNIT OFFERING PRICE REGISTRATION FEE
- -------------------------------------------------------------------------------------------
<S>                       <C>               <C>            <C>            <C>
Common Stock, par value
 $.01 per share of UAL.   14,463,093 shares      (1)            (1)       $700,462.52(1)(2)
- -------------------------------------------------------------------------------------------
Series B Preferred Stock
 of UAL................     35,985 shares        (1)            (1)              (1)
- -------------------------------------------------------------------------------------------
Depositary Preferred
 Shares representing the
 Series B Preferred
 Stock.................   35,984,175 shares      (1)            (1)              (1)
- -------------------------------------------------------------------------------------------
Series D Redeemable
 Preferred Stock of
 UAL...................     28,927 shares        (1)            (1)              (1)
- -------------------------------------------------------------------------------------------
Series A Senior
 Debentures due 2004 of
 United................     $449,802,200        (1)(2)         (1)(2)          (1)(2)
- -------------------------------------------------------------------------------------------
Series B Senior
 Debentures due 2014 of
 United................     $449,802,200        (1)(2)         (1)(2)          (1)(2)
</TABLE>

- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------
(1) This Registration Statement covers the shares of Common Stock, par value
    $0.01 per share, of UAL (the "New Shares") and the shares of Series D
    Redeemable Preferred Stock of UAL to be issued in exchange for, and upon
    conversion of the shares of the Common Stock, par value $5 per share, of
    UAL (the "Old Shares") in connection with the Recapitalization.
    Immediately upon issuance, the Series D Redeemable Preferred Stock will be
    redeemed for (i) $25.80 in cash, and (ii) either (a) Series A Debentures
    due 2004 of United, Series B Debentures due 2014 of United (collectively,
    the "Debentures") and Depositary Preferred Shares representing shares of
    Series B Preferred Stock of UAL, (b) the cash proceeds from the sale of
    such securities or (c) a mixture of cash proceeds and securities. For the
    purposes of calculating the registration fee pursuant to Rule 457(f)(1),
    (i) the number of Old Shares to be exchanged and converted is 28,926,185
    and (ii) $746,295,573 of cash ($25.80 per Old Share), which will be paid
    by UAL in connection with the Recapitalization, has been subtracted
    (pursuant to Rule 457(f)(3)) from the aggregate market value of Old Shares
    to be exchanged and converted in the Recapitalization. The aggregate
    market value of the Old Shares has been computed by taking the average of
    the high and low prices for the Old Shares on the New York Stock Exchange,
    Inc. on April 6, 1994 ($127.125).
(2) As noted below, the Debentures were registered as Debt Securities of
    United pursuant to the Registration Statement on Form S-3 (No. 33-57192)
    filed on January 21, 1993. Of the aggregate fee calculated pursuant to
    Rule 457(f), $310,208.41 is attributable to the Debentures, and the amount
    of the registration fee has been reduced by such amount. The net
    registration fee was paid upon the original filing.
 
                                ---------------
 
  THE REGISTRANTS HEREBY AMEND THIS REGISTRATION STATEMENT ON SUCH DATE OR
DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANTS
SHALL FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS
REGISTRATION STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH
SECTION 8(A) OF THE SECURITIES ACT OF 1933 OR UNTIL THE REGISTRATION STATEMENT
SHALL BECOME EFFECTIVE ON SUCH DATE AS THE COMMISSION, ACTING PURSUANT TO SAID
SECTION 8(A), MAY DETERMINE.
 
  AS PROVIDED UNDER RULE 429, THE DEBENTURES TO BE OFFERED HEREUNDER WERE
REGISTERED AS DEBT SECURITIES OF UNITED PURSUANT TO THE REGISTRATION STATEMENT
ON FORM S-3 (NO. 33-57192) FILED ON JANUARY 21, 1993.
 
- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------

<PAGE>
 

PART II - INFORMATION NOT REQUIRED IN PROSPECTUS

Item 21.  Exhibits and Financial Statement Schedules.

UAL Corporation ("UAL") and United Air Lines, Inc. ("United") filed the
Registration Statement on Form S-4 on April 12, 1994, and Amendments Numbers 1
through 3 thereto were filed on April 21, 1994, May 27, 1994 and June 10, 1994,
respectively.  The Registration Statement on Form S-4 became effective on June
10, 1994.  This Post-Effective Amendment No. 1 to the Registration State-ment on
Form S-4 includes as exhibits material distribut-ed by UAL reflecting the final
terms of the securities issued in connection with the recapitalization of UAL.


Exhibit No.         Document
- --------------      --------------------------------

99.1                Advertisement in the Wall Street Journal, National
                    Edition, July 1, 1994, page C11.

99.2                Mailgram sent to Stockholders of UAL Corporation dated
                    July 1, 1994.

99.3                Press Release dated as of June 30, 1994.

99.4                Press Release dated as of June 30, 1994.

99.5                Opinion of CS First Boston Cor-poration dated July 1,
                    1994.

99.6                Opinion of Lazard Freres & Co. dated July 1, 1994.

<PAGE>
 

                                 SIGNATURES

     Pursuant to the requirements of the Securities Act, the registrant has duly
caused this registration statement to be signed on its behalf by the
undersigned, thereunto duly authorized, in Elk Grove Township, Illinois, on the
1st day of July, 1994.

                            UAL Corporation
                            
 
                            By            /s/ John C. Pope
                               -----------------------------------------
                                 John C. Pope, President and
                                   Chief Operating Officer

     Pursuant to the requirements of the Securities Act of 1933, this
registration statement has been signed by the following persons in the
capacities noted on this 1st day of July, 1994.


<TABLE> 
<CAPTION> 
     Signature                     Title                                                Date
     ---------                     -----                                                ----
<S>                              <C>                                               <C> 


         *                       Director, and Chairman and                         July 1, 1994
- -----------------------           Chief Executive Officer 
  Stephen M. Wolf                                
             
 

  /s/ John C. Pope               Director, and President and Chief                  July 1, 1994
- -----------------------           Operating Officer (principal    
      John C. Pope                accounting officer and principal
                                  financial officer)              
                              

         *                       Director                                           July 1, 1994
- ------------------------
    Neil A. Armstrong


         *                       Director                                           July 1, 1994 
- -------------------------             
    Andrew F. Brimmer


         *                       Director                                           July 1, 1994 
- -------------------------             
    Richard P. Cooley


         *                       Director                                           July 1, 1994 
- -------------------------             
    Carla A. Hills


         *                       Director                                           July 1, 1994 
- --------------------------             
     Fujio Matsuda

</TABLE>
 
                                       3

<PAGE>
 

<TABLE> 
<CAPTION> 


     Signature                     Title                           Date
     ---------                     -----                           ----
<S>                              <C>                           <C> 
         *                       Director                      July 1, 1994 
- -------------------------                          
  John F. McGillicuddy                             
                                                   
                                                   
          *                      Director                      July 1, 1994 
- -------------------------                          
     Harry Mullikin                                
                                                   
                                                   
          *                       Director                      July 1, 1994 
- -------------------------                          
   James J. O'Connor                               
                                                   
                                                   
          *                       Director                      July 1, 1994 
- -------------------------                          
   Frank A. Olson                                  
                                                   
                                                   
           *                       Director                      July 1, 1994 
- -------------------------                          
    Ralph Strangis                                 
                                                   
                                                   
           *                       Director                      July 1, 1994 
- -------------------------
   Paul E. Tierney, Jr.


</TABLE>
 

*  By        /s/ John C. Pope
     ------------------------------------
           John C. Pope
        (Attorney-In-Fact)

                                       4

<PAGE>
 
                                   SIGNATURES

     Pursuant to the requirements of the Securities Act, the registrant has duly
caused this registration statement to be signed on its behalf by the
undersigned, thereunto duly authorized, in Elk Grove Township, Illinois, on the
1st  day of July, 1994.

                            United Air Lines, Inc.
                        
 
                            By         /s/ John C. Pope
                               -----------------------------------------
                                 John C. Pope, President and
                                   Chief Operating Officer

     Pursuant to the requirements of the Securities Act of 1933, this
registration statement has been signed by the following persons in the
capacities noted on this 1st day of July, 1994.

<TABLE>
<CAPTION>
 
Signature                          Title                                    Date
- ---------                          -----                                    ----
<S>                             <C>                                    <C>
 
             *                  Director, and Chairman and               July 1, 1994
- -----------------------------    Chief Executive Officer
      Stephen M. Wolf            (principal executive officer)   
                                   
 
    /s/ John C. Pope             Director, President and                 July 1, 1994
- -----------------------------     Chief Operating Officer
      John C. Pope                (principal financial officer)
                                   
 
              *                  Director, and Executive Vice            July 1, 1994
- -----------------------------     President--Corporate Affairs
     Lawrence M. Nagin            and General Counsel           
                                   
 
              *                  Director, and Executive Vice            July 1, 1994
- -----------------------------     President--Marketing and
      James M. Guyette            Planning                 
                                   
 
              *                  Director, and Senior Vice               July 1, 1994
- -----------------------------     President--Human Resources
       Paul G. George             
 

              *                  Director, and Executive Vice            July 1, 1994
- -----------------------------     President--Operations
   Joseph R. O'Gorman, Jr.     


              *                  Vice President and Controller           July 1, 1994
- -----------------------------     (principal accounting officer)
     Frederic F. Brace         

</TABLE>
 

*  By     /s/ John C. Pope
       ----------------------
              John C. Pope
           (Attorney-In-Fact)


                                      5

<PAGE>
 

                                EXHIBIT INDEX
                                -------------


<TABLE> 
<CAPTION> 
Exhibit No.                Document
- --------------             --------------------------------
<S>                        <C> 
99.1                       Advertisement in the Wall Street Jour-
                           nal, National Edition, July 1, 1994, 
                           page C11.

99.2                       Mailgram sent to Stockholders of UAL 
                           Corporation dated July 1, 1994.

99.3                       Press Release dated as of June 30, 1994.

99.4                       Press Release dated as of June 30, 1994.

99.5                       Opinion of CS First Boston Corporation 
                           dated July 1, 1994.

99.6                       Opinion of Lazard Freres & Co. dated 
                           July 1, 1994.
</TABLE>
 


                                      6





<PAGE>

                                                                  Exhibit 99.1

 
                               UAL Corporation
                           is pleased to announce
                       to Holders of its Common Stock
                   the following information regarding its
                          Proposed Recapitalization
                                 __________

If the transactions relating to the proposed recapitalization (the
"Recapitalization") of UAL Corporation ("UAL") are approved by vote of the
holders (the "Stockholders") of the common stock, par value $5 per share, of UAL
("Old Shares"), UAL expects that United Air Lines, Inc. will consummate the
public offering (the "United Series A Offering") of its Series A Debentures due
2004 (the "Series A Debentures") and the public offering  (the "United Series B
Offering" and, together with the United Series A Offering, the "United Debt
Offerings") of its Series B Debentures due 2014 (the "Series B Debentures") and
UAL expects to consummate the public offering of its depositary shares (the
"Depositary Shares") representing interests in its Series B Preferred Stock,
without par value, ("Public Preferred Stock") of the Company (the "UAL Preferred
Offering" and, together with the United Debt Offerings, the "Offerings").  If
all of the Offerings are consummated, Stockholders will receive in exchange for
each Old Share in the Recapitalization, one half of a share of common stock, par
value $0.01 per share,
 of UAL and a cash payment of $84.81, of which $46.81 will
result from the Offerings.  Notwithstanding the foregoing, there can be no
assurance that any of the Offerings will be consummated.

If the United Series A Offering is not consummated, Stockholders will receive,
in lieu of the cash proceeds from the United Series A Offering, $15.55 principal
amount of Series A Debentures bearing an interest rate of 10.125% and the cash
payment in respect of each Old Share will be reduced by $15.05.  If the United
Series B Offering is not consummated, Stockholders will receive, in lieu of the
cash proceeds from the United Series B Offering,  $15.55 principal amount of
Series B Debentures bearing an interest rate of 10.825% and the cash payment in
respect of each Old Share will be reduced by $15.08.  If that the UAL Preferred
Offering is not consummated, Stockholders will receive, in lieu of the cash
proceeds from the UAL Preferred Offering, Depositary Shares representing
interests in $17.96 liquidation preference of  Public Preferred Stock bearing a
dividend rate of 11.375% and the cash payment in respect of each Old Share will
be reduced by $16.68.  If the Stockholders approve the Recapitalization and none
of the Offerings is consummated, Stockholders will receive, in lieu of the cash
proceeds from the Offerings, $15.55 principal amount of Series A Debentures,
$15.55 principal amount of Series B Debentures, Depositary Shares representing
interests in $17.96 liquidation preference of Public Preferred Stock and a cash
payment of $38.00.

<PAGE>
 
                The Meeting of Stockholders is July 12, 1994.

At the Meeting, Stockholders will vote on, among other things, the
Recapitalization of UAL.  If you were a Stockholder as of close of business on
June 9, 1994 and you have not yet returned your proxy card in connection with
the upcoming Meeting, UAL urges that you do so as soon as possible.

                       We urge you to vote your shares
                  in favor of the Recapitalization of UAL.
               Please Complete, Sign and Mail Your Proxy Card.

    If you have any questions regarding the Recapitalization of UAL, please 
    call 1 (800) 223-2064.

For more complete information regarding the Recapitalization of UAL, please
refer to the Proxy Statement/Joint Prospectus of UAL Corporation and United Air
Lines, Inc. and the exhibits thereto that were sent to Stockholders of UAL
Corporation on June 10, 1994 and the Supplement to such Proxy Statement/Joint
Prospectus expected to be sent to Stockholders of UAL Corporation on July 2,
1994.  Copies of the Proxy Statement/Joint Prospectus are available, and copies
of the Supplement to such Proxy Statement/Joint Prospectus will be available, to
Stockholders free of charge by calling 1(800) 223-2064.

                                      2



<PAGE>

                                                                  Exhibit 99.2

 
                                                July 1, 1994


Dear UAL Stockholder:

     This is to inform you that we expect to consummate public offerings of
United Air Lines, Inc. Series A Debentures due 2004, United Series B Debentures
due 2014 and Depositary Shares representing interests in UAL Corporation Series
B Preferred Stock.  If such offerings and UAL's proposed recapitalization are
consummated, each share of UAL common stock will be exchanged for one-half of a
share of new UAL common stock and $84.81 in cash (attributable to $38 from the
company's cash, $15.05 from the sale of Series A Debentures, $15.08 from the
sale of Series B Debentures and $16.68 from the sale of Depositary Shares).

     If these public offerings are not consummated for any reason, UAL
stockholders will receive one-half of a share of new UAL common stock, $38 in
cash, $15.55 principal amount of United 10.125% Series A Debentures, $15.55
principal amount of United 10.825% Series B Debentures and depositary shares
representing interests in $17.96 liquidation preference of UAL 11.375% preferred
stock if the recapitalization is consummated.

     The vote of UAL stockholders on the recapitalization will occur on July
12, 1994.  IF YOU HAVE NOT YET RETURNED YOUR PROXY CARD, UAL URGES THAT YOU DO
SO AS
 SOON AS POSSIBLE.

                                                UAL CORPORATION



<PAGE>
 
                                                                  Exhibit 99.3


                                    
                                    
                                            Corporate Communications Contacts:
                                            John Kiker    (708) 952-4162
                                            Joe Hopkins   (708) 952-5770        
                                            Tony Molinaro (708) 952-4971      
                                            Night/Weekend (708) 952-4088      
                                                      
                                            Investor Relations Contact:       
                                            Pamela Hanlon (708) 952-7501        

              UAL CORPORATION MODIFIES ESOP RECAPITALIZATION PLAN


FOR IMMEDIATE RELEASE
- ---------------------

     CHICAGO, June 30, 1994 -- UAL Corporation today announced that it has
modified its Agreement and Plan of Recapitalization to provide for the use of
$300 million of available Company cash in substitution for a portion of the
preferred stock which the Company was to issue upon closing of its proposed
recapitalization.

     Under the agreed modifications, existing stockholders would receive in the
recapitalization the following for each share of UAL common stock:

     .    one-half of a share of new UAL common stock;

     .    $38.00 in cash from the Company's existing cash resources, an
          increase of $12.20 per share; 

     .    the cash proceeds from the sale in the previously 


                                   (more)

<PAGE>

                                    -2- 

          announced public offering of $31.10 principal amount of United debt
          securities (or, if this public offering is not consummated, the same
          principal amount of debt securities);

     .    the cash proceeds from the sale in the previously announced public
          offering of depositary shares representing
 interests in $18.90 
          liquidation preference (reflecting a $12.20 per share decrease) of
          the company's preferred stock (or, if this public offering is not
          consummated, the same amount of preferred stock.)

          The cash proceeds from the sale of the United debt securities and/or
the UAL preferred stock will be subject to reduction, in accordance with the
Agreement and Plan of Recapitalization as modified, if the final pricing for
such instrument exceeds certain maximum rates.

          The company will file shortly a Report on Form 8-K with the SEC
containing pro forma financial information giving effect to the foregoing
modification.

                                   (more)

<PAGE>
 
                                     -3-

          UAL also announced that it had reached a tentative understanding to 
settle shareholder litigation pending in the Delaware Chancery Courts relating 
to the Agreement and Plan of Recapitalization.

          Consummation of the recapitalization remains subject to satisfaction
of certain conditions, including a stockholder vote scheduled for July 12 in
Chicago.

                                    -UA-



<PAGE>
 
                                                                  EXHIBIT 99.4

news                                                           UNITED AIRLINES


                                            Corporate Communications Contacts:
                                            John Kiker          (708) 952-4162
                                            Joe Hopkins         (708) 952-5770
                                            Tony Molinaro       (708) 952-4971
                                            Night               (708) 952-4088

                                            Investor Relations Contact:
                                            Pamela Hanlon       (708) 952-7501


                      UAL CORP. BEGINS PUBLIC OFFERING


FOR IMMEDIATE RELEASE
- ---------------------

     CHICAGO, June 30, 1994 -- UAL Corporation and United Air Lines, Inc. 
announced today that they have commenced their public offerings of 
$370,200,000 aggregate principal amount of United 10.67% Series A Debentures 
due 2004, $371,000,000 aggregate principal amount of United's 11.21% Series B 
Debentures due 2014 and 16,416,000 Depositary Shares representing interests in
$410,400,000 liquidation preference of UAL's Series B Preferred Stock. The 
Series B Preferred Stock will bear dividends at the annual rate of 12-1/4%. 
The offering price on the Depositary Shares is $25.00 per share; each 
Depositary Share represents $25 liquidation preference of Series B Preferred 
Stock.

                                   (more)

<PAGE>
 
                                     -2-

     The offerings are being made in connection with the proposed
recapitalization of UAL, and the proceeds of the offerings will be used to
fund a portion of the cash consideration payable to holders of UAL's current
common stock in UAL's proposed recapitalization, together
 with $38 per share
in cash from the Company's existing cash resources. UAL holders will receive
one-half of a new share of UAL common stock and $84.81 in cash per share if
the offerings and the recapitalization are consummated.

     The offering of the United Debentures is being underwritten by Merrill
Lynch & Co., CS First Boston, Goldman Sachs & Co. and Salomon Brothers Inc and
the UAL Preferred Offering is being underwritten through a group that is being
managed by Merrill Lynch & Co., A.G. Edwards & Sons, Inc., PaineWebber
Incorporated, Prudential Securities Incorporated, Goldman, Sachs & Co., Smith
Barney Inc., Donaldson, Lufkin & Jenrette, Kidder, Peabody & Co., Bear Stearns
& Co. Inc., Lehman Brothers and Morgan Stanley & Co. Incorporated.


                                   (more)


<PAGE>
 
                                     -3-

       The vote by UAL's stockholders on the proposed recapitalization is
scheduled to occur on July 12. The closing of the offerings is scheduled to
occur the same day, subject to a favorable vote and certain other conditions.
       The terms of the Agreement and Plan of the Recapitalization provide 
that if the underwritings are not consummated for any reason, UAL stockholders
would receive one-half of a share of a new UAL common stock, $38 in cash, 
$15.55 principal amount of United 10 1/8% Series A Debentures, $15.55 
principal of United 10.825% Series B Debentures and depositary shares 
representing interests in $17.96 liquidation preference of 11 3/8% UAL 
preferred stock if the recapitalization was consummated.

                                    -UA-



<PAGE>
 
                                                                  Exhibit 99.5



July 1, 1994



Board of Directors
UAL Corporation
1200 East Algonquin Road
Elk Grove Township, IL 60007


Gentlemen and Madam:

You have requested our opinion as to the fairness, from a financial point of
view, to the holders (the "Common Stockholders") of shares of common stock, par
value $5.00 per share ("Old Shares"), of UAL Corporation, a Delaware corporation
("UAL"), of the consideration to be received by such Common Stockholders in
connection with the proposed recapitalization of UAL (the "Transaction"), as set
forth in, and pursuant to the terms of, the Agreement and Plan of
Recapitalization dated as of March 25, 1994, as  amended and restated (the
"Recapitalization Agreement"), among UAL and the Airline Pilots Association,
International, and the International Association of Machinists and Aerospace
Workers (together with other participating employees, the "Participating
Employees").

We understand that the Transaction, as more specifically set forth in the
Recapitalization Agreement, provides that, in exchange for certain labor
concessions by the Participating Employees, UAL will issue common stock to
certain employee trusts/ESOPs equal to a minimum of 55% and a maximum of 63% of
the common stock of UAL.
  We also understand that in the Transaction the current
Common Stockholders of UAL will receive, for each Old Share held, one-half of a
new share of common stock, par value $.01 per share, of UAL (representing an
equity interest immediately after the Transaction is completed of approximately
45% of one Old Share's current percentage equity interest in UAL, subject to
possible reduction) and $84.81 in cash.  A portion of the cash consideration to
be received by  Common Stockholders of UAL will represent the gross proceeds of
a public offering by United Air Lines, Inc. of its Series A Debentures due 2004
and its Series B Debentures due 2014 and the gross proceeds of a public offering
by UAL of  Depositary Preferred Shares representing shares of its Series B
Preferred Stock, without par value.

<PAGE>
 
                                                                             2

In arriving at our opinion, we have reviewed and analyzed the Recapitalization
Agreement, as well as certain publicly available business and financial
information relating to UAL.  We have also reviewed certain other information,
including financial forecasts provided to us by UAL.  We have met with UAL's
management to discuss the past and current operations and financial condition
and prospects of UAL.  We have also considered certain financial and stock
market data for UAL and we have compared that data with similar data for other
publicly held companies in businesses similar to those of UAL, and we have
considered the financial terms of certain other business combinations that have
recently been effected.  We also considered such other information, financial
studies, analyses and investigations and financial, economic and market criteria
that we deemed relevant.  In addition, we have reviewed the alternative of not
effecting a reorganization or similar transaction and UAL implementing various
operating strategies considered by it which, if fully implemented, might result
in a greater value to Common Stockholders than the Transaction; however, we
understand and have assumed for purposes of this opinion that the Board of
Directors of UAL has determined, in light of various factors relating to the
implementation of such operating strategies and the availability of the
Transaction, not to pursue such implementation.

In connection with our review, we have not independently verified any of the
foregoing information and have relied on its being complete and accurate in all
material respects.  With respect to the financial forecasts, we have assumed
that they have been reasonably prepared on bases reflecting the best currently
available estimates and judgments of UAL's management as to the future financial
performance of UAL.  We express no view as to such forecasts or the assumptions
on which they are based.  We have not made an independent evaluation or
appraisal of the assets or liabilities of UAL, nor have we been furnished with
any such appraisals.  We were not requested to, and did not, solicit third party
offers to acquire all or any part of UAL, nor, to our knowledge, has any
interest in making such an offer been presented by any third party, including in
response to the public disclosure regarding discussions between UAL and the
Participating Employees.  We have assumed that the results expected by UAL's
management to be obtained from the Transaction, including those arising from the
Participating Employees' labor concessions, will be realized.  Our opinion is
necessarily based solely upon information available to us and business, market,
economic and other conditions as they exist on, and can be evaluated as of, the
date hereof.  Our opinion does not address UAL's underlying business decision to
effect the Transaction.

We have acted as financial advisor to UAL in connection with the Transaction and
will receive a fee for our services, a significant portion of which is
contingent upon the consummation of the Transaction.  We will also receive a fee
for rendering this opinion and other additional services currently being
rendered to UAL.  In the ordinary course of our business, we actively trade the
debt and equity securities of UAL for our own account and for the accounts of
customers and, accordingly, may at any time hold a long or short position in
such securities.

It is understood that this opinion is only for the information of the Board of
Directors of UAL.  However, this opinion may be included in its entirety in any
proxy statement from UAL to its Common Stockholders. This opinion may not,
however, be summarized, 

<PAGE>
 
                                                                             3

excerpted from or otherwise publicly referred to without our prior written
consent, which will not unreasonably be withheld. In addition, we may not be
otherwise publicly referred to without our prior consent, which will not
unreasonably be withheld.

Based upon and subject to the foregoing, it is our opinion that, as of the date
hereof, the consideration to be received by the Common Stockholders of UAL in
the Transaction, taken as a whole, is fair to such Common Stockholders from a
financial point of view.


Very truly yours,

CS FIRST BOSTON CORPORATION



By:  _______________________________



<PAGE>
 
                                                                  Exhibit 99.6



July 1, 1994



Board of Directors
UAL Corporation
1200 East Algonquin Road
Elk Grove Township, IL 60007


Gentlemen and Madam:

You have requested our opinion as to the fairness, from a financial point of
view, to the holders (the "Common Stockholders") of shares of common stock, par
value $5.00 per share ("Old Shares"), of UAL Corporation, a Delaware corporation
("UAL"), of the consideration to be received by such Common Stockholders in
connection with the proposed recapitalization of UAL (the "Transaction"), as set
forth in, and pursuant to the terms of, the Agreement and Plan of
Recapitalization dated as of March 25, 1994, as amended and restated (the
"Recapitalization Agreement"), among UAL and the Airline Pilots Association,
International, and the International Association of Machinists and Aerospace
Workers (together with other participating employees, the "Participating
Employees").

We understand that the Transaction, as more specifically set forth in the
Recapitalization Agreement, provides that, in exchange for certain labor
concessions by the Participating Employees, UAL will issue common stock to
certain employee trusts/ESOPs equal to a minimum of 55% and a maximum of 63% of
the common stock of UAL.
  We also understand that in the Transaction the current
Common Stockholders of UAL will receive, for each Old Share held, one-half of a
new share of common stock, par value $.01 per share, of UAL (representing an
equity interest immediately after the Transaction is completed of approximately
45% of one Old Share's current percentage equity interest in UAL, subject to
possible reduction) and $84.81 in cash.  A portion of the cash consideration to
be received by Common Stockholders of UAL will represent the gross proceeds of a
public offering by United Air Lines, Inc.  of its Series A Debentures due 2004
and its Series B Debentures due 2014 and the gross proceeds of a public offering
by UAL of Depositary Preferred Shares representing shares of its Series B
Preferred Stock, without par value.

<PAGE>
 
                                                                             2

In arriving at our opinion, we have reviewed and analyzed the Recapitalization
Agreement, as well as certain publicly available business and financial
information relating to UAL.  We have also reviewed certain other information,
including financial forecasts provided to us by UAL.  We have met with UAL's
management to discuss the past and current operations and financial condition
and prospects of UAL.  We have also considered certain financial and stock
market data for UAL and we have compared that data with similar data for other
publicly held companies in businesses similar to those of UAL, and we have
considered the financial terms of certain other business combinations that have
recently been effected.  We also considered such other information, financial
studies, analyses and investigations and financial, economic and market criteria
that we deemed relevant.  In addition, we have reviewed the alternative of not
effecting a reorganization or similar transaction and UAL implementing various
operating strategies considered by it which, if fully implemented, might result
in a greater value to Common Stockholders than the Transaction; however, we
understand and have assumed for purposes of this opinion that the Board of
Directors of UAL has determined, in light of various factors relating to the
implementation of such operating strategies and the availability of the
Transaction, not to pursue such implementation.

In connection with our review, we have not independently verified any of the
foregoing information and have relied on its being complete and accurate in all
material respects.  With respect to the financial forecasts, we have assumed
that they have been reasonably prepared on bases reflecting the best currently
available estimates and judgments of UAL's management as to the future financial
performance of UAL.  We express no view as to such forecasts or the assumptions
on which they are based.  We have not made an independent evaluation or
appraisal of the assets or liabilities of UAL, nor have we been furnished with
any such appraisals.  We were not requested to, and did not, solicit third party
offers to acquire all or any part of UAL, nor, to our knowledge, has any
interest in making such an offer been presented by any third party, including in
response to the public disclosure regarding discussions between UAL and the
Participating Employees.  We have assumed that the results expected by UAL's
management to be obtained from the Transaction, including those arising from the
Participating Employees' labor concessions, will be realized.  Our opinion is
necessarily based solely upon information available to us and business, market,
economic and other conditions as they exist on, and can be evaluated as of, the
date hereof.  Our opinion does not address UAL's underlying business decision to
effect the Transaction.

We have acted as financial advisor to UAL in connection with the Transaction and
will receive a fee for our services, a significant portion of which is
contingent upon the consummation of the Transaction.  A portion of this fee
relates to the rendering of this opinion.

It is understood that this opinion is only for the information of the Board of
Directors of UAL.  However, this opinion may be included in its entirety in any
proxy statement from UAL to its Common Stockholders.  This opinion may not,
however, be summarized, excerpted from or otherwise publicly referred to without
our prior written consent, which will not unreasonably be withheld.  In
addition, we may not be otherwise publicly referred to without our prior
consent, which will not unreasonably be withheld.

<PAGE>
 
                                                                             3

Based upon and subject to the foregoing, it is our opinion that, as of the date
hereof, the consideration to be received by the Common Stockholders of UAL in
the Transaction, taken as a whole, is fair to such Common Stockholders from a
financial point of view.


Very truly yours,



_______________________________
Lazard Freres & Co.