Amgen Inc. has only one product to sell, but when it's the latest biotechnology blockbuster drug, one can be enough.

Called erythropoietin, or EPO, it knocks out chronic anemia in patients with kidney disease. Like insulin for diabetics, EPO is their only medical alternative.

The drug's success has vaulted the Thousand Oaks company to the top tier of the biotechnology industry. At the moment Amgen enjoys a monopoly on a $300-million-a-year business. EPO profits are piling up at the rate of nearly $50 million annually.

While Wall Street has stumbled this year, Amgen's stock has more than doubled. On deck, Amgen has a second drug designed to help cancer patients ward off infections. Analysts say it will be an even bigger winner than EPO.

Is there anything wrong with this cheery diagnosis? Maybe so.

In the next month or two, a federal appeals court in Washington should release its decision in a critical case involving rival EPO patents held by Amgen and Genetics Institute, a Cambridge, Mass., concern. The decision could secure the EPO market for Amgen. Or, Amgen could lose its rights to the drug. Or, as most analysts expect, the court could effectively force Amgen to share the EPO business and leave it vulnerable to a hefty damages suit.

Jeffrey R. Swarz, an analyst with the investment firm of Goldman, Sachs & Co., said that if the appeals court upholds both EPO patents, Genetics Institute will promptly sue Amgen for "past and present damages in excess of $100 million." Jim McCamant, editor of the Medical Technology Stock Letter in Berkeley, expects the court decision to go against Amgen and to knock 20% off Amgen's stock price.

Amgen must also worry about damages it may have to pay a Johnson & Johnson subsidiary, which helped fund Amgen's EPO research and owns the rights to sell the drug for all anemia treatments except severe kidney disease. J&J maintains that it was deprived of profits because Amgen botched J&J's chances for quick government approval to sell the drug. A Chicago arbitration judge is reviewing the case.

Swarz believes that "J&J has asked for very significant damages of between $100 million and $200 million."

"In a worst-case scenario, Amgen may be looking at $300 million in damages" to settle both EPO cases, Swarz said. Granted, Swarz and other analysts conceded, the final financial damage to Amgen should be much less. But for now the only certainty is uncertainty.