EXHIBIT 99.1

         OSI PHARMACEUTICALS COMPLETES ITS ACQUISITION OF CELL PATHWAYS

MELVILLE, N.Y.--(BUSINESS WIRE)--June 12, 2003--OSI Pharmaceuticals, Inc.
(NASDAQ: OSIP) announced today that the acquisition of Cell Pathways, Inc.
became effective today in a stock-for-stock merger valued at approximately $31
million based on the average closing prices of OSI's common stock around the
time of the announcement of the acquisition. Under the terms of the transaction,
OSI exchanged .0567 shares of OSI common stock for each share of Cell Pathways
common stock. OSI provided additional consideration in the form of a five-year
contingent value right, through which each share of Cell Pathways common stock
will be eligible for an additional .040 of a share of OSI common stock in the
event of a filing of an NDA (new drug application) for either of the two
newly-acquired clinical candidates, CP461 or Aptosyn(R) (exisulind).

Through this transaction, OSI has added a technology platform designed to
selectively induce apoptosis, or programmed cell death through the sustained
activation of protein kinase G in pre-cancerous and cancerous cells. OSI gained
two drug candidates in clinical development: CP461, a potent, second-generation
molecule that is currently being evaluated in dose-ranging Phase I studies and a
series of exploratory Phase II studies in chronic lymphocytic leukemia, renal
cell carcinoma and prostate cancer and Aptosyn(R), an earlier-generation
molecule that is currently in Phase III trials in combination with Taxotere(R)
for the treatment of advanced non-small cell lung cancer. The acquisition of
Cell Pathways also provides OSI with an additional oncology product,
Gelclair(TM), a bioadherent oral gel that provides relief for the treatment of
pain associated with oral mucositis, a debilitating side effect often seen in
cancer patients undergoing radiation treatment or chemotherapy. OSI is
establishing a commercial operation to support Gelclair(TM) and Novantrone(R)
(mitoxantrone concentrate for injection), which the Company markets exclusively
for its approved oncology indications in the United States.

"The Cell Pathways apoptosis technology platform complements our existing R&D
approaches in this area and provides the opportunity to advance the development
of novel anti-cancer drugs which we believe may have value both as monotherapy
agents and in combination with existing chemotherapy drugs or other novel,
targeted agents," stated Colin Goddard, Ph.D., Chief Executive Officer of the
Company. "In addition, this transaction gives us our second marketed product in
Gelclair(TM), which addresses a major unmet need in cancer treatment today, that
of oral mucositis. We are on schedule to re-launch both Gelclair(TM) and the
recently licensed Novantrone(R) under the OSI brand later this year."

OSI Pharmaceuticals is a leading biotechnology company focused on the discovery,
development and commercialization of high-quality, next-generation oncology
products that both extend and improve the quality-of-life for cancer patients
worldwide. OSI has a balanced pipeline of oncology drug candidates that includes
both next-generation cytotoxic

agents and novel mechanism-based, gene-targeted therapeutics focused in the
areas of signal transduction and apoptosis. OSI's most advanced drug candidate,
Tarceva(TM) (erlotinib HCl), a small-molecule inhibitor of the HER1 gene, is
currently in Phase III clinical trials for lung and pancreatic cancers. OSI has
a commercial presence in the U.S. oncology market where it exclusively markets
Novantrone(R) (mitoxantrone concentrate for injection) for approved oncology
indications.

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,
pharmaceutical collaborators' ability 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.