The present study describes 403 patients with thrombosis, from a uniform ethnic and geographical background. Two-hundred-and-seven individuals had suffered mild or moderate stroke and 196 individuals suffered venous thromboembolism. We recorded levels of antithrombin, protein C and protein S, plasminogen and plasma homocysteine, and the presence of the factor V Leiden mutation, the prothrombin 20210G-->A variant, and the methylenetetrahydrofolate reductase (MTHFR) 677C-->T polymorphism. Controls for the mutation frequencies consisted of Guthrie card blood spots from a cohort of new-born babies. The cumulative prevalence of deficiencies in antithrombin, protein C, protein S or plasminogen was 2.4% in patients with stroke and 11.2% in patients with venous thrombosis. The factor V Leiden mutation was present in 11.1% of patients with stroke and 26.5% of patients with venous thrombosis, compared with 6.6% of controls (n = 4188; P < 0.05 and P < 0.0001, respectively). The prevalence of the prothrombin 20210A variant was 3.1% in patients with venous thrombosis, 1.9% in patients with stroke and 2.0% in controls (n = 500; P > 0.05). Hyperhomocysteinemia was present in 16.0% of patients with stroke and 17.6% of patients with venous thrombosis. The prevalence of the MTHFR 677T/T genotype was no different in patients with stroke (10.6%) and venous thrombosis (8.7%) than in controls (8.3%; n = 1084; P > 0.05); thus, it apparently contributed to thrombosis only via its influence on total plasma homocysteine, which was significantly increased in patients with the T/T genotype (P < 0.001). The MTHFR T/T genotype did not further increase the risk for thrombosis in carriers of the factor V Leiden mutation. Overall, thrombotic events were associated with a known risk factor in 27% of patients with stroke and 55% of patients with venous thrombosis.