Antithrombin III (AT III) was determined in 290 patients with deep venous thrombosis and/or pulmonary embolism by immunological methods (radial immunodiffusion, Laurell technique) and by biological activity (heparin cofactor activity and anti-Xa activity). Patients with venous thrombosis had a significantly lower AT III concentration, as determined by the immunological methods or biological method (heparin cofactor activity), than normal persons without any history of venous thrombosis. A decreased level of AT III was found in 27 patients. In these patients the immunoreactive antithrombin III was decreased to the same degree as biological activity (heparin cofactor activity or anti-Xa activity). Thirteen out of these 27 patients belonged to 9 families and, hence, congenital AT III deficiency can be assumed in these cases. The aetiology was unknown in the other half. Patients with AT III deficiency are prone to spontaneous and/or recurrent venous thrombosis. A high incidence of pulmonary embolism and particularly, of fatal pulmonary embolism is remarkable. In more than half of the patients the first thrombotic event occurred before the age of 35. The treatment of choice in such patients is with oral anticoagulants of the coumarin group.