<PAGE>

                                                                     EXHIBIT 4.5

                                                                  EXECUTION COPY

                            ACCOUNT CONTROL AGREEMENT

                  ACCOUNT CONTROL AGREEMENT (this "AGREEMENT") dated as of
September 8, 2003, among OSI Pharmaceuticals, Inc., a Delaware corporation (the
"PLEDGOR"), The Bank of New York, a New York banking corporation, as Trustee
(the "SECURED PARTY"), and The Bank of New York, a New York banking corporation,
as securities intermediary and depository bank (the "ACCOUNT HOLDER").

PRELIMINARY STATEMENTS:

                  (1)      The Pledgor has granted the Secured Party a security
interest (the "SECURITY INTEREST") in certain security entitlements (the
"PLEDGED SECURITY ENTITLEMENTS") with respect to certain U.S. Treasury
securities (the "PLEDGED FINANCIAL ASSETS") identified on Schedule I attached
hereto, as well as certain funds maintained by the Pledgor with the Account
Holder and carried from time to time in an account with the Account Holder, ABA
No. 021000018, Account No. 341436 at its office at 101 Barclay Street, New York,
New York 10286, in the name of "OSI Pharmaceuticals Pledge Account" (the
"ACCOUNT") and all additions thereto and substitutions and proceeds thereof,
pursuant to, and as more particularly described in, a Collateral Pledge and
Security Agreement dated as of September 8, 2003, among the Pledgor and the
Trustee (as the same may hereafter be amended, supplemented or otherwise
modified from time to time, the "PLEDGE AGREEMENT"; terms defined in the Pledge
Agreement and not otherwise defined herein being used herein as therein
defined). The Pledgor acknowledges having received value for such pledge.

                  (2)      Terms defined in Article 8 or 9 of the Uniform
Commercial Code in effect in the State of New York ("N.Y. UNIFORM COMMERCIAL
CODE") are used in this Agreement as such terms are defined in such Article 8 or
9.

                  (3)      The Pledgor, the Trustee and the Account Holder are
delivering this Agreement pursuant to the terms of Section 4(f) of the Pledge
Agreement.

                  NOW, THEREFORE, in consideration of the premises and of the
mutual agreements contained herein, the parties hereto hereby agree as follows:

                  SECTION 1. The Accounts. The Account Holder represents and
warrants to, and agrees with, the Secured Party that:

                  (a)      The Account Holder maintains the Account for the
         Pledgor, and all the property (including, without limitation, funds and
         Pledged Financial Assets) and all additions thereto and substitutions
         and proceeds thereof held by the Account Holder for the account of the
         Pledgor are, and will continue to be, credited to the Account.

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                                       2

                  (b)      To the extent that funds are credited to the Account,
         the Account is a deposit account; and to the extent that financial
         assets are credited to the Account, the Account is a securities
         account. The Account Holder is (i) the bank with which the Account is
         maintained and (ii) the securities intermediary with respect to
         financial assets held in the Account. The Pledgor is (x) the Account
         Holder's customer with respect to the Account and (y) the entitlement
         holder with respect to financial assets credited from time to time to
         the Account.

                  (c)      Notwithstanding any other agreement to the contrary,
         the Account Holder's jurisdiction with respect to the Account for
         purposes of the N.Y. Uniform Commercial Code is, and will continue to
         be for so long as the Security Interest shall be in effect, the State
         of New York.

                  (d)      Attached as Exhibit A hereto is the schedule of the
         property credited to the Account as of the date hereof showing the
         property credited to the Account.

                  (e)      The Account Holder does not know of any claim to or
         interest in the Account or any property (including, without limitation,
         funds and financial assets) credited to the Account, except for claims
         and interests of the parties referred to in this Agreement.

                  SECTION 2. Control by Secured Party. The Account Holder will
comply with (i) all instructions directing disposition of the funds in the
Account, (ii) all notifications and entitlement orders that the Account Holder
receives directing it to transfer or redeem any financial asset in the Account,
and (iii) all other directions concerning the Account, including, without
limitation, directions to distribute to the Secured Party proceeds of any such
transfer or redemption or interest or dividends on property in the Account (any
such instruction, notification or direction referred to in clause (i), (ii) or
(iii) above being an "ACCOUNT DIRECTION"), in each case of clauses (i), (ii) and
(iii) above originated by the Secured Party without further consent by the
Pledgor or any other Person. The Account Holder will comply with Account
Directions and other directions concerning the Account originated by, and only
by, the Secured Party. The Account Holder acknowledges that the Secured Party
has exclusive control over the Account and all Pledged Financial Assets
contained therein from time to time.

                  SECTION 3. Priority of Secured Party's Security Interest. (a)
The Account Holder (i) subordinates to the Security Interest and in favor of the
Secured Party any security interest, lien, or right of recoupment or setoff that
the Account Holder may have, now or in the future, against the Account or any
property (including, without limitation, any funds and financial assets)
credited to the Account, and (ii) agrees that it will not exercise any right in
respect of any such security interest or lien or any such right of recoupment or
setoff until the Security Interest is terminated, except that the Account Holder
(A) will retain its prior security interest and lien on property credited to the
Account, (B) may exercise any right in respect of such security interest or
lien, and (C) may exercise any right of recoupment or setoff against the
Account, in the case of clauses (A), (B) and (C) above, to secure or to satisfy,
and only to secure or to satisfy, payment (x) for such property, (y) for its
customary fees and expenses for the routine maintenance and operation of the
Account, and (z) if the Account is a deposit account, for the face amount of any
items that have been credited to the Account but are subsequently returned
unpaid because of uncollected or insufficient funds.

                  (b)      The Account Holder will not enter into any other
         agreement with any Person relating to Account Directions or other
         directions with respect to the Account.

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                                       3

                  SECTION 4. Statements, Confirmations, and Notices of Adverse
Claims. (a) The Account Holder will send copies of all statements and
confirmations for the Account simultaneously to the Secured Party and the
Pledgor.

                  (b)      When the Account Holder knows of any claim or
         interest in the Account or any property (including, without limitation,
         funds and financial assets) credited to the Account other than the
         claims and interests of the parties referred to in this Agreement, the
         Account Holder will promptly notify the Secured Party and the Pledgor
         of such claim or interest.

                  SECTION 5. The Account Holder's Responsibility. (a) The
Account Holder will be liable to the Secured Party for complying with Account
Directions or other directions concerning the Account from the Pledgor or any
Person other than the Secured Party.

                  (b)      The Account Holder will not be liable to the Pledgor
         or the Secured Party for complying with an Account Direction or other
         direction concerning the Account originated by the Secured Party, even
         if the Pledgor notifies the Account Holder that the Secured Party is
         not legally entitled to issue the Account Direction or such other
         direction unless the Account Holder takes the action after it is served
         with an injunction, restraining order, or other legal process enjoining
         it from doing so, issued by a court of competent jurisdiction, and had
         a reasonable opportunity to act on the injunction, restraining order or
         other legal process.

                  (c)      This Agreement does not create any obligation of the
         Account Holder except for those expressly set forth in this Agreement
         and, to the extent that the Account is a securities account, in Part 5
         of Article 8 of the N.Y. Uniform Commercial Code and, to the extent
         that the Account is a deposit account, in Article 4 of the N.Y. Uniform
         Commercial Code. In particular, the Account Holder need not investigate
         whether the Secured Party is entitled under the Secured Party's
         agreements with the Pledgor to give an Account Direction or other
         direction concerning the Account. The Account Holder may rely on
         notices and communications it believes given by the appropriate party.

                  SECTION 6. Indemnity. The Pledgor will indemnify the Trustee
and the Account Holder and their respective officers, directors, employees and
agents against claims, liabilities and expenses arising out of this Agreement
(including, without limitation, reasonable attorney's fees and disbursements),
except to the extent the claims, liabilities or expenses are caused by the
Trustee's or the Account Holder's (as applicable) gross negligence or willful
misconduct as found by a court of competent jurisdiction in a final,
non-appealable judgment.

                  SECTION 7. Termination; Survival. (a) This Agreement shall
terminate automatically upon receipt by the Account Holder of written notice
executed by two officers of the Trustee that (i) all of the Secured Obligations
have been paid in full in cash or otherwise satisfied or (ii) all of the
Collateral has been released, which ever is earlier, and the Account Holder
shall thereafter be relieved of all duties and obligations hereunder.

                  (b)      The Account Holder may terminate this Agreement on 60
         days' prior notice to the Secured Party and the Pledgor, provided that
         before such termination the Account Holder and the Pledgor shall make
         arrangements to transfer the property (including, without limitation,
         all funds and financial assets) credited to the Account to another
         Account Holder that shall

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                                       4

         have executed, together with the Pledgor, a control agreement in favor
         of the Secured Party in respect of such property in substantially the
         form of this Agreement or otherwise in form and substance satisfactory
         to the Secured Party.

                  (c)      Sections 5 and 6 will survive termination of this
         Agreement.

                  SECTION 8. Governing Law. This Agreement and the Account will
be governed by the law of the State of New York. The Account Holder and the
Pledgor may not change the law governing the Account without the Secured Party's
express prior written agreement.

                  SECTION 9. Entire Agreement. This Agreement is the entire
agreement, and supersedes any prior agreements, and contemporaneous oral
agreements, of the parties concerning its subject matter.

                  SECTION 10. Amendments. No amendment of, or waiver of a right
under, this Agreement will be binding unless it is in writing and signed by the
party to be charged.

                  SECTION 11. Financial Assets. The Account Holder agrees with
the Secured Party and the Pledgor that, to the fullest extent permitted by
applicable law, all property (other than funds) credited from time to time to
the Account will be treated as financial assets under Article 8 of the N.Y.
Uniform Commercial Code.

                  SECTION 12. Notices. A notice or other communication to a
party under this Agreement will be in writing (except that Account Directions
may be given orally), will be sent to the party's address set forth under its
name below or to such other address as the party may notify the other parties
and will be effective on receipt.

                  SECTION 13. Binding Effect. This Agreement shall become
effective when it shall have been executed by the Pledgor, the Secured Party and
the Account Holder, and thereafter shall be binding upon and inure to the
benefit of the Pledgor, the Secured Party and the Account Holder and their
respective successors and assigns.

                  SECTION 14. Execution in Counterparts. This Agreement may be
executed in any number of counterparts and by different parties hereto in
separate counterparts, each of which when so executed shall be deemed to be an
original and all of which taken together shall constitute one and the same
agreement. Delivery of an executed counterpart of a signature page to this
Agreement by telecopier shall be effective as delivery of an original executed
counterpart of this Agreement.

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                                       5

                  IN WITNESS WHEREOF, the parties hereto have caused this
Agreement to be executed by their respective officers thereunto duly authorized,
as of the date first above written.

                                             OSI PHARMACEUTICALS, INC.

                                             By /s/
                                                ------------------------------
                                                Name:
                                                Title:

                                             Address:
                                             58 South Service Road
                                             Melville, NY 11747

                                             THE BANK OF NEW YORK, as Trustee

                                             By /s/
                                                ------------------------------
                                                Name:
                                                Title:

                                             Address:
                                             101 Barclay Street, Floor 8W
                                             New York, NY 10286

                                             THE BANK OF NEW YORK, as Account
                                              Holder

                                             By /s/
                                                ------------------------------
                                                Name:
                                                Title:

                                             Address:
                                             101 Barclay Street, Floor 8W
                                             New York, NY 10286

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                                   SCHEDULE I

<TABLE>
<CAPTION>
                                                                                                     FACE AMOUNT
       DESCRIPTION OF ASSET                                                CUSIP NUMBER                OF ASSET
       --------------------                                                ------------                --------
<S>                                                                        <C>                      <C>
UNITED STATES TREAS NT STRP PRIN PMT                                         912820BH9              USD 2,193,000
UNITED STATES TREAS NT STRP PRIN PMT                                         912820BK2              USD 2,193,000
UNITED STATES TREAS NT STRP PRIN PMT                                         912820BM8              USD 2,193,000
US TREASURY BD STRIPPED                                                      912803AG8              USD 2,193,000
US TREASURY BD STRIPPED                                                      912803AJ2              USD 2,193,000
US TREAS NTS SEC STRIPPED                                                    912833CQ1              USD 2,193,000
</TABLE>