The ruling, good news for Bristol-Meyers Squibb and Sanofi-Aventis, which co-markets the blockbuster product, was not unexpected. U.S. District Judge Sidney H. Stein indicated last fall he leaned toward upholding the patent, which is not scheduled to expire until 2011.

Besides upholding the Plavix patent, Stein permanently prohibited generic manufacturing company Apotex from selling additional supplies of its version of the drug, called clopidogrel bisulphate.

Stein said he would schedule a hearing to set damages to be paid by Apotex, which flooded the market with more than $1 billion worth of its product last fall.

Apotex, based in Toronto, said it would appeal Tuesday's ruling. The company's chief executive, Bernard C. Sherman, said in a statement he thought the patent ultimately would be held invalid.