A healthy man with previously undiagnosed, mild von Willebrand's disease received one 1 of 6 percent hydroxyethyl starch solution intravenously. Following infusion, the bleeding time lengthened, platelet adhesion decreased, and the partial thromboplastin time was prolonged in association with decreased plasma levels of factor VIII coagulant activity, factor VIII-related antigen, and factor VIII ristocetin cofactor. Abnormalities persisted for days, but overt bleeding did not occur. Care should be taken to screen and possibly reject prospective granulocyte donors with positive personal or family bleeding histories. Caution should be used when administering hydroxyethyl starch as a colloidal plasma volume expanding agent to patients with underlying hemostatic defects.