An autosomally transmitted bleeding diathesis sharing some, but not all, features previously described in von Willebrand's disease (vWd) was studied in five patients representing three generations of a single family. Bleeding times in the upper normal range in conjunction with low-normal platelet counts, normal factor VIII coagulant activity and VIII-related antigen, decreased VIII-ristocetin cofactor activity, selective decrease of the higher molecule weight factor VIII/von Willebrand factor (VIII/vWF) multimers, and increased ristocetin-induced platelet agglutination at low ristocetin concentrations were characteristic. Binding of patient VIII/vWF to washed normal platelets was within normal limits, whereas binding of normal VIII/vWF to patient platelets was significantly increased (p less than 0.001 at 0.6 mg/ml ristocetin). This disorder accordingly appears to involve an intrinsic platelet abnormality affecting platelet-VIII/vWF interactions. It is proposed that the concept of vWD be broadened to include patients with this abnormality, which may appropriately be called "Platelet-type von Willebrand's disease."