Ninety-two new cases of hemophilia A and 16 of hemophilia B are reported. The data, together with those from other reported series, suggest that hemophilia A and B comprise over 90% of all congenital hemorrhagic diseases due to defective formation of a fibrin clot. Of those with deficiencies of AHF or PTC, the incidence of PTC deficiency is about 16%. The rise in age at death of patients with hemophilia from 14.5 to 29 years since 1940 is largely attributed to the wide use and availability of fresh blood and its derivatives. The usefulness of various coagulation tests in the differential diagnosis of the congenital abnormalities of the clotting mechanism is demonstrated. With the development of more sensitive diagnostic techniques, the concept of hemophilia has broadened to include many cases of very mild clinical disease in patients with essentially normal life expectancy. Specific assay procedures are necessary for diagnosis.