The relationship between platelet function and plasma free fatty acid concentration has been studied serially during the initial 24 h in 11 patients suffering from acute myocardial infarction and in 5 patients with severe angina pectoris, Similar results were obtained in the 2 groups. Plasma free fatty acid concentration was initially high, and decreased significantly. The distribution of plasma free fatty acids remained unchanged. Platelet concentration was constant, whereas the percentage of reversible venous platelet aggregates initially was higher than in 11 healthy persons matched for age and sex. Platelet aggregates decreased transiently at 16 h. Venous reversible platelet aggregates correlated significantly with concentration of plasma free fatty acid, thus establishing a possible link between a change in lipid metabolism and platelet function. Plasma concentration of platelet factor 4 increased slightly but significantly during the initial hours. This may indicate an increased platelet release reaction.