EXHIBIT 99.1

                                             NEWS RELEASE

Contact:   OSI Pharmaceuticals, Inc.         Burns McClellan (representing OSI)
           Kathy Galante                     Jonathan Nugent (Investors)
           Director                          Kathy Jones, Ph.D. (Media)
           Investor / Public Relations       212-213-0006
           631-962-2000

          OSI PHARMACEUTICALS ANNOUNCES ISSUANCE OF US PATENT COVERING
     COMBINATION THERAPY USING DIPEPTIDYL PEPTIDASE IV (DPIV) INHIBITORS AND
                            OTHER ANTIDIABETIC AGENTS

          - PATENT LICENSED TO OSI'S UK SUBSIDIARY PROSIDION LIMITED -

MELVILLE, NY - MAY 11TH, 2005 - OSI Pharmaceuticals, Inc. (NASDAQ: OSIP),
announced today the issuance of US Patent No. 6,890,898 which claims combination
therapy for modifying glucose metabolism comprising administration of a DPIV
inhibitor and a further therapeutic agent e.g. metformin. OSI has an exclusive
sub-licensable license under US Patent No. 6,890,898 and corresponding
equivalents worldwide for the use of non-boronic acid DPIV inhibitors through
its UK-based subsidiary, Prosidion Limited. The patent arises out of research
from the laboratories of Drs. William Bachovchin, Tufts University; Andrew
Plaut, Tufts-New England Medical Center Hospitals; and Daniel Drucker, Toronto
General Hospital (University Health Network), University of Toronto. The Company
has granted a number of non-exclusive sub-licenses under this patent family and
expects to grant additional non-exclusive licenses in the future.

The Company acquired its DPIV technology platform from Probiodrug AG in June
2004. In addition to a Phase II compound, PSN9301, the platform comprises a
portfolio of DPIV medical use patents. These include issued patents and pending
patent applications corresponding to US 6,303,661, US 6,890,898 and WO 01/72290,
with claims covering DPIV as a target for anti-diabetes therapy and the use of
combinations of DPIV inhibitors with other anti-diabetic drugs.

"Inhibition of DPIV is one of the most promising new strategies for the
treatment of type 2 diabetes and combination therapy could represent a
significant component of the market for DPIV inhibitors. The grant of this
patent strengthens the portfolio of DPIV assets acquired by Prosidion last
year," stated Anker Lundemose, M.D., Ph.D., President of OSI's diabetes and
obesity business unit, (OSI) Prosidion. "Pursuing additional license grants
under this patent portfolio in return for near-term revenues and potential
royalties is an ongoing component of Prosidion's business strategy."

ABOUT (OSI)

OSI Pharmaceuticals is committed to shaping medicines and changing patients'
lives by discovering, developing and commercializing high-quality and novel
pharmaceutical products that extend life or improve the quality of life for
cancer and diabetes patients worldwide. The company operates through two
business teams, (OSI) Oncology and (OSI) Prosidion. (OSI) Oncology is focused on
developing molecular targeted therapies designed to change the paradigm of
cancer care. (OSI) Prosidion is committed to the generation of novel, targeted
therapies for the treatment of type 2 diabetes and obesity. OSI's flagship
product, Tarceva(TM) (erlotinib), is the first drug discovered and developed by
OSI to obtain FDA approval and the only EGFR inhibitor to have demonstrated the
ability to improve survival in both non-small cell lung cancer and pancreatic
cancer patients. OSI markets Tarceva(TM) through partnerships with Genentech
Inc. in the U.S. and with Roche throughout the rest of the world.

(OSI) Prosidion is the diabetes and obesity business group within OSI
Pharmaceuticals, dedicated to the discovery and development of novel drugs for
the treatment of type 2 diabetes and obesity. (OSI) Prosidion's lead compound
PSN9301 is a Dipeptidyl Peptidase IV (DPIV) inhibitor currently in Phase II
clinical trials. Other products targeting glycogen phosphorylase inhibition and
glucokinase activation are scheduled to enter Phase I clinical trials in 2005.
(OSI) Prosidion owns or has licensing rights to a portfolio of DPIV medical use
patents with claims covering DPIV as a target for anti-diabetes therapy and the
use of combinations of DPIV inhibitors with other anti-diabetes drugs such as
metformin. A number of non-exclusive licenses to the patent estate have been
granted to major pharmaceutical companies. (OSI) Prosidion operates through
OSI's wholly-owned subsidiary, Prosidion Limited, in Oxford, UK.

This news release contains forward-looking statements. These statements are
subject to known and unknown risks and uncertainties that may cause actual
future experience and results to differ materially from the statements made.
Factors that might cause such a difference include, among others, the completion
of clinical trials, the FDA review process and other governmental regulation,
Prosidion's and OSI's and their collaborators' abilities to successfully develop
and commercialize drug candidates, competition from other pharmaceutical
companies, the ability to effectively market products and other factors
described in OSI Pharmaceuticals' filings with the Securities and Exchange
Commission. PSN9301, PSN105, PSN357 and PSN010 are investigational compounds and
have not yet been approved as safe or efficacious in humans for their ultimate
intended use.