A patient with benign monoclonal lgG lambda paraproteinemia, acquired von Willebrand syndrome (AVWS), and chronic melena successfully responding to high-dose intravenous immunoglobulin (lvlg) is reported. Coagulation parameters at admission were APTT (ratio) 1.68; VIII:C 11 IU/dL; vWF:Ag 7 IU/dL: Ricof <3 IU/dl. RIPA was >1.8 mg/ml, and bleeding time (BT) was prolonged (18 min). No evidence for an in vitro inhibitor against the VIII/vWF complex was observed. VIII/vWF measurements showed a short-lived increase after both DDAVP and Hemate P, and BT was transiently normalized. After intravenous lg (1 g/kg for 2 days), VIII/vWF measurements, hemostatic parameters and multimeric pattern were completely corrected (VIII/C 106 IU/dl, vWF:Ag 168 IU/dl, RiC of 147 IU/dl, APTT ratio 0.89, BT 5′), with a return to pre-infusion values after 15 days. Hemoccult test became negative and packed red cell transfusions, of which 130 units were administered during the last year, were no longer required. After 18 months the patient is on maintenance treatment with repeated courses of Ig, at 3 to 4-week intervals based on VIII/vWF and BT monitoring. © 1992 Wiley-Liss, Inc.