A sequence variation in the 3'-untranslated region of the prothrombin (PT) gene (20210 G-->A) was recently claimed to be associated with elevated plasma prothrombin levels and an increased risk for venous and arterial thrombosis. We examined the prevalence of the 20210 A allele in the prothrombin gene in 400 healthy controls and in 263 patients with proven premature atherosclerotic disease. In addition, we measured prothrombin, prothrombin fragment 1 + 2, thrombin-antithrombin (TAT) complex and D-dimer levels in plasma from carrier and non-carrier patients. The frequency of the variant allele was 1% in the control subjects and 2.7% in the patient group, yielding a relative risk (RR) for the 20210 A allele of 2.7 (95% CI 0.8-9.4). All heterozygotes in the patient group were found to have had a myocardial infarction (MI), yielding a RR for MI of 4.2 (95% CI 1.2-14.6). Plasma prothrombin levels in carriers (126+/-10) were higher than in non-carriers (103+/-1, P=0.02). The levels of TAT complexes (16+/-9 v 6+/-1 microg/ml, P=0.02) as well as of prothrombin fragment 1 + 2 (1.5+/-0.3 v 1.0+/-0.1 nmol/l, P=0.02) were also elevated in carriers of the mutation. Our findings suggest that the 20210 G-->A mutation in the prothrombin gene is a genetic risk factor for MI. In addition, our data provide evidence for an association of the mutation with excessive thrombin generation, which may contribute to the understanding of its role in venous and arterial disease.