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                                  FORM 10-Q/A

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

(Mark One)

[X]      QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
         SECURITIES EXCHANGE ACT OF 1934

For the quarterly period ended                  March 31, 2000                 .
                              -------------------------------------------------

                                       OR

[ ]      TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
         SECURITIES EXCHANGE ACT OF 1934

For the transition period from                to
                               --------------    ---------------------.

Commission file number                          0-15190
                       ---------------------------------------------------------

                            OSI Pharmaceuticals, Inc.
- --------------------------------------------------------------------------------
             (Exact name of registrant as specified in its charter)

              Delaware                                    13-3159796
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            (State or other jurisdiction of               (I.R.S. Employer
            incorporation or organization)                Identification No.)

    106 Charles Lindbergh Boulevard, Uniondale, New York         11553
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            (Address of principal executive offices)           (Zip Code)

                                  516-222-0023
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              (Registrant's telephone number, including area code)

- --------------------------------------------------------------------------------
              (Former name, former address and former fiscal year,
                         if changed since last report.)

Indicate by check mark whether the registrant (1) has filed all reports required
to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during
the preceding 12 months (or for such shorter period that the registrant was
required to file such reports), and (2) has been subject to such filing
requirements for the past 90 days. Yes   X     No
                                      ------      ------


                      APPLICABLE ONLY TO CORPORATE ISSUERS:

At April 30, 2000 the registrant had outstanding 26,419,659 shares of common
stock, $.01 par value.

This Form 10-Q/A is being filed to amend Exhibit 10.1 of the Form 10-Q of the
registrant for the quarter ended March 31, 2000, filed on May 15, 2000, to
disclose certain information that was previously redacted.

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                                   PART II

ITEM 6.     EXHIBITS AND REPORTS ON FORM 8-K

            (a)   EXHIBITS

                  3.1   Certificate of Incorporation, as amended (1)

                  3.2   Amended and Restated By-Laws (2)

                *10.1   License Agreement, dated as of January 3, 2000,
                        by and between the Company and American Home
                        Products Corporation and American Cyanamid Company

                 10.2   Yeast Technology Agreement, dated as of February
                        15, 2000, by and between the Company and Cadus
                        Pharmaceutical Corporation (3)

                   27   Financial Data Schedule (3)

                 -----------------------------

                   (1)  Included as an exhibit to the Company's quarterly
                        report on Form 10-Q for the quarter ended March 31,
                        1999, filed on May 14, 1999, and incorporated herein
                        by reference.

                   (2)  Included as an exhibit to the Company's current report
                        on Form 8-K, filed on January 8, 1999, and incorporated
                        herein by reference.

                   (3)  Included as an exhibit to the Company's quarterly report
                        on Form 10-Q for the quarter ended March 31, 2000, filed
                        on May 15, 2000.

                    *   Portions of this exhibit have been redacted and are the
                        subject of a confidential treatment request filed with
                        the Secretary of the Securities and Exchange Commission
                        pursuant to Rule 24b-2 under the Securities Exchange Act
                        of 1934, as amended.

            (b)   REPORTS ON FORM 8-K

                  The Company filed a current report on Form 8-K on March 1,
                  2000 with the Securities and Exchange Commission via EDGAR,
                  pertaining to the private sale of 3.325 million shares of
                  common stock. The earliest event covered by the report
                  occurred on February 25, 2000.




                                      -18-
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                                   SIGNATURES

            Pursuant to the requirements of the Securities Exchange Act of 1934,
the registrant has duly caused this Form 10-Q/A to be signed on its
behalf by the undersigned thereunto duly authorized.

                              OSI PHARMACEUTICALS, INC.



Date:   June 19, 2000          /s/ Robert L. Van Nostrand
                              ------------------------------------------
                              Name: Robert L. Van Nostrand
                              Title: Vice President and Chief Financial Officer,
                                     Secretary and Treasurer




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                                  EXHIBIT INDEX



            Exhibit No.                  Description
            -----------                  -----------
                   
               3.1       Certificate of Incorporation, as amended (1)

               3.2       Amended and Restated By-Laws (2)

             *10.1       License Agreement, dated as of January 3, 2000, by and between the
                         Company and American Home Products Corporation and American
                         Cyanamid Company

              10.2       Yeast Technology Agreement, dated as of February 15, 2000, by and
                         between the Company and Cadus Pharmaceutical Corporation (3)

                27       Financial Data Schedule (3)

         ------------------------------

               (1)       Included as an exhibit to the Company's quarterly report on Form 10-Q for the quarter ended
                         March 31, 1999, filed on May 14, 1999, and incorporated herein by reference.

               (2)       Included as an exhibit to the Company's current report on Form 8-K, filed on January 8,
                         1999, and incorporated herein by reference.

               (3)       Included as an exhibit to the Company's quarterly report on Form 10-Q for the quarter ended
                         March 31, 2000, filed on May 15, 2000.

                 *       Portions of this exhibit have been redacted and are the subject of a confidential treatment
                         request filed with the Secretary of the Securities and Exchange Commission pursuant to Rule
                         24b-2 under the Securities Exchange Act of 1934, as amended.