Although eicosapentaenoic acid (EPA) has been shown to have beneficial effects in the prevention of atherosclerosis, the mechanism by which these effects occur is not entirely clear. We investigated the lipid composition of platelets in paired subjects with and without atherosclerotic disease, either hypercholesterolemic (low density lipoprotein [LDL] cholesterol [Chol] greater than or equal to 170 mg/dl) or normocholesterolemic (LDL-Chol less than 170 mg/dl). Platelets from patients with atherosclerotic disease had a lower phosphatidylcholine (PC)/Chol ratio, when compared with those from patients without atherosclerotic disease, irrespective of LDL-Chol levels. Eleven patients with atherosclerotic disease were treated with purified EPA ethyl ester (1.8 g/day), and changes in lipid composition of platelets were investigated. Plasma levels of total Chol and LDL-Chol decreased significantly after EPA administration. The phospholipid (PL)/Chol ratio and the PC/Chol ratio in platelets from patients with atherosclerotic disease increased significantly after 4-10 wk EPA treatment. The EPA content in platelets increased, while the arachidonic acid (AA) content decreased. EPA-induced changes in the PL/Chol and the PC/Chol ratios of platelets, as well as fatty acyl chain shifts, may be related to the beneficial effects in preventing atherosclerosis, possibly by increase in the membrane fluidity.