The primary cause of the defective hemostasis in hemophilia is the lack of thromboplastinogen in the blood. This anomaly is congenital and hereditary. The mechanism whereby the concentration of thromboplastinogen is depressed in the blood is not known and no satisfactory explanation can be given why the defect manifests itself only in the male. The degree of the hypothromboplastinogenemia determines the intrinsic severity of the disease. The diagnosis is based on establishing the existence of a bleeding tendency from birth, in demonstrating a deficiency of thromboplastinogen in the blood, and in finding a typical hereditary pattern.