Antithrombin III (ATIII) deficiency is inherited as an autosomal dominant trait. Three types of ATIII deficiency are recognized clinically. The prevalence of ATIII deficiency is uncertain; it has been estimated to occur in between one in 2,000 and one in 20,000 subjects. ATIII deficiency is found in between 4 and 6 percent of young patients with venous thrombosis, similar to but slightly lower than the prevalence of protein C and protein S deficiency in young subjects with thrombosis. The chances of finding a deficiency is increased if there is a history of familial or recurrent venous thrombosis. Cross-sectional reports in the literature are that between 30 and 80 percent of carriers have thrombosis. Thrombosis is uncommon in the first decade, but the risk rises sharply between the ages of 15 and 30. The major clinical manifestations of ATIII deficiency are young age at onset, idiopathic thrombosis, family history, and recurrent venous thromboembolism. Pregnancy and surgery are predisposing factors. Approaches to prophylaxis and treatment are discussed.