The clinical picture of antithrombin III (AT-III) deficiency is characterized by early manifestations of serious venous thromboembolism. The inheritance is autosomal dominant. The precipitating factors of the clinical signs are infections, trauma and pregnancy. For a correct diagnosis, application of various methods (immunological, amidolytic, electrophoresis) are needed. There are several types of AT-III defect; they are characterized by a decrease in amount and function, a functional decrease, or a pathological heparin-binding. Adequate treatment of thromboembolic episodes caused by AT-III deficiency is a lifelong coumarin therapy. In pregnancy and after surgery heparin can only be given together with AT-III substitution. The decreased AT-III activity may change during treatment and this has a diagnostic importance.