Two families were studied because of a hemorrhagic tendency. The presumptive diagnosis of von Willebrand’s disease was suggested by low factor VIII levels (7.5%-33%), prolonged template Ivy bleeding time (9.5-17 min), low platelet adhesiveness (0%-8%), normal platelet factor 3, and normal clot retraction. Further studies, however, showed abnormal platelet aggregation with ADP, epinephrine, and collagen, and deficient release of platelet antiheparin activity and 14C serotonin. Patients’ platelets, rendered free of plasma by gel filtration, continued to show abnormal aggregation when resuspended in normal plasma. Plasma from the patients contained greater than 200% factor VIII by immunologic assay. The patients’ coagulant factor VIII level returned to base line within 24 hr after plasma or cryoprecipitate transfusions. Unlike von Willebrand’s disease, management of these patients required both platelets and cryoprecipitate to prevent bleeding.