Antithrombin is the major inhibitor of blood coagulation, primarily inactivating thrombin and factor Xa. Inherited antithrombin deficiency is associated with an increased risk of thromboembolism. Its prevalence is estimated to be about 0.2% in the general healthy population, whereas it is 2% in patients with a history of thrombosis. We previously demonstrated thrombosis patients receiving therapeutic dosage of low-molecular-weight heparin having a lower antifactor Xa activity than originally estimated. In those patients, mutations in the AT gene (SERPINC1) were detected despite normal antithrombin activity. Thus we concluded that prevalence of antithrombin mutations might be higher than previously calculated. The aim of the present study was to investigate the prevalence of hereditary antithrombin mutations by analyzing the antithrombin gene in patients with a history of thromboembolism. Mutation analyses were performed by direct sequencing of SERPINC1 in 150 patients (aged <40 years) with thromboembolism and normal antithrombin levels. Patients with thrombophilic defects [factor V Leiden (G1691A), prothrombin (G20210T) mutation, protein C deficiency, protein S deficiency, and antiphospholipid antibodies] were excluded. The results were compared with those of 150 healthy controls without any personal history of thrombosis. Mutation analyses of all seven antithrombin exons revealed five (3.5%) patients with antithrombin mutations: three different heterozygous missense mutations were found in exon 2 and one (in two patients) in exon 7. Prevalence of inherited antithrombin mutations in thrombosis patients is higher than previously estimated. Functional antithrombin tests are unable to detect all clinically relevant antithrombin defects.