Elderly people ingested 150 mg/day of icosapentaenoic acid (20:5n-3) or a placebo for one month. Platelet aggregation, platelet arachidonate metabolism and the fatty acid composition of both plasma and platelet lipids were investigated before and after the intake. Platelet aggregation induced by collagen, epinephrine or low concentrations of ADP was significantly reduced after 20:5n-3 intake. Besides, the main oxygenated product formation from endogenous platelet arachidonate under thrombin stimulation was markedly decreased after the 20:5n-3 supplementation. Such a decrease was absent after placebo. Moreover, no modification in the fatty acid composition of both plasma lipids and platelet phosphatidylcholine could be observed. We conclude that intake of low amounts of 20:5n-3 by elderly people, is able to lower their platelet sensitivity to aggregating agents, probably by decreasing the endogenous formation of platelet thromboxane A2, although no modification in the fatty acid composition was detected.