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                UNITED STATES SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                                   FORM 10-K/A
                                 Amendment No. 1


(Mark One)

/X/   Annual Report Pursuant to Section 13 or 15(d) of the Securities
      Exchange Act of 1934
      For the fiscal year ended        September 30, 2004        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
--------------------------------------------     ----------------------------
      (State or other Jurisdiction of                   (I.R.S. Employer
       Incorporation or Organization)                  Identification No.)

    58 South Service Road, Melville, N.Y.                  11747
--------------------------------------------     -------------------------
  (Address of Principal Executive Offices)               (Zip Code)

       Registrant's Telephone Number, including area code (631) 962-2000
                                                          --------------
           Securities Registered Pursuant to Section 12(b) of the Act:

    Title of each class               Name of each exchange on which registered
          None                                          None
---------------------------------    -------------------------------------------

           Securities Registered Pursuant to Section 12(g) of the Act:
                   Common Stock, par value $.01 per share, and
        Series SRPA Junior Participating Preferred Stock Purchase Rights
        ----------------------------------------------------------------
                                (Title of Class)

       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
                                              ----          ----

       Indicate by check mark if disclosure of delinquent filers pursuant to
Item 405 of Regulation S-K is not contained herein, and will not be contained,
to the best of Registrant's knowledge, in definitive proxy or information
statements incorporated by reference in Part III of this Form 10-K or any
amendment to this Form 10-K. / /

       As of March 31, 2004 the aggregate market value of the Registrant's
voting stock held by non-affiliates was $1,140,265,997. For purposes of this
calculation, shares of common stock held by directors, officers and stockholders
whose ownership exceeds five percent of the common stock outstanding at March
31, 2004 were excluded. Exclusion of shares held by any person should not be
construed to indicate that the person possesses the power, direct or indirect,
to direct or cause the direction of the management or policies of the
Registrant, or that the person is controlled by or under common control with the
Registrant.

       Indicate by check mark whether the Registrant is an accelerated filer (as
defined in Rule 12b-2 of the Securities Exchange Act of 1934).  Yes  X   No
                                                                    ----   ----

       As of December 1, 2004, there were 50,634,509 shares of the Registrant's
common stock, par value $.01 per share, outstanding.

                       DOCUMENTS INCORPORATED BY REFERENCE

       Portions of the Registrant's definitive proxy statement for its 2005
annual meeting of stockholders are incorporated by reference into Part III of
this Form 10-K.


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                                EXPLANATORY NOTE

       OSI Pharmaceuticals, Inc. is filing this amendment to the Form 10-K for
the year ended September 30, 2004, filed with the Securities and Exchange
Commission on December 14, 2004, to amend the cover sheet by removing the check
mark from the box relating to Item 405 of Regulation S-K. Otherwise, the Form
10-K is not amended in any way by this Amendment.



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                                   SIGNATURES

       Pursuant to the requirements of Section 13 or 15(d) of the Securities
Exchange Act of 1934, as amended, the Registrant has duly caused this Amendment
to be signed on its behalf by the undersigned, thereunto duly authorized.


                                   OSI PHARMACEUTICALS, INC.



                                   By:/s/ Robert L. Van Nostrand
                                      ------------------------------
                                      Robert L. Van Nostrand
                                      Vice President and Chief Financial Officer

Date: January 18, 2005



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                                INDEX TO EXHIBITS


Exhibit

31.1    Certification of Chief Executive Officer pursuant to Rule 13a-14(a) or
        15(d)-14(a).

31.2    Certification of Chief Financial Officer pursuant to Rule 13a-14(a) or
        15d-14(a).

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




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