Abstract Platelets play an important role in the development of atherosclerosis. The arachidonic acid, whose oxygenated metabolites are potent regulators of the platelet-vessel wall interactions, is released from membrane phospho-lipids by the phospholipase(s) system(s). These membrane-linked phenomena are strongly modulated by the membrane physical properties. The present study was carried out to investigate the relationship between membrane fluidity and arachidonic acid metabolism in platelets from atherosclerotic patients. Twenty-one patients with peripheral vascular disease and twelve controls were studied. Platelets from patients showed an increase in membrane fluidity and enhanced thrombin-stimulated thromboxane synthesis. No alterations were found, however, in total phospholipid fatty acid composition. A significant decrease in the cholesterol/phospholipid ratio could account for the alterations in the membrane physical properties described in the platelets from patients.