A 69-year-old woman presented with splenic marginal zone lymphoma associated with acquired von Willebrand's disease (AVWD). Laboratory abnormalities included markedly decreased plasma levels of factor VIII coagulant (C) activity (VIII:C 28%), von Willebrand's factor (VWF) antigen (Ag) (vWF:Ag < 6%), and VWF ristocetin cofactor (RCo) activity (VWF:RCo, < 12%). VWF multimer analysis revealed a severe type II defect. Treatment with cryoprecipitate, high-dose gamma globulin or desmopressin given intravenously was unsuccessful. Clinical bleeding and coagulation abnormalities showed transient improvement after replacement therapy with Humate-P concentrate. The coagulation abnormalities improved partially after splenectomy and completely after subsequent chemotherapy. The neoplastic lymphocytes in the blood and spleen strongly expressed platelet glycoprotein Ib (CD42) and VWF but not other platelet-associated antigens.