Aspirin (ASA) is widely used in the treatment of cardiovascular diseases. Recently, we have found that aspirin decreases not only platelet aggregation but also thrombin generation. This effect, however, was seen only in certain subjects. Therefore we decided to examine influence of a single dose of aspirin (500 mg) on thrombin generation in healthy volunteers. Thrombin genesis was assessed by serial measurements of fibrinopeptide A concentration in blood emerging from standardised forearm skin incisions. Aspirin reduced thrombin generation in persons with normal serum level of lipids. This effect was lost, however, in subjects with high level of cholesterol and lipoprotein (a)--well known risk factors of ischaemic heart disease. While the mechanism by which aspirin affects thrombin generation remains to be elucidated, our data indicate that hypercholesterolemic subjects might benefit less than others from preventive aspirin treatment.