The clinical and laboratory findings of a patient with a bleeding disorder, laboratory features of von Willebrand's disease, and a monoclonal gammopathy are described. There was no evidence of von Willebrand's disease in his five children. Laboratory features of von Willebrand's disease in the patient included a long bleeding time, low factor VIII level by one- and two-stage clotting assays and by immunoassay, decreased platelet glass adhesiveness using native and heparinized blood, and correction of platelet adhesiveness by addition of cryoprecipitate in vitro. Attempts to implicate the monoclonal IgG in the pathogenesis of the von Willebrand's syndrome were unsuccessful using the following tests: immunoprecipitation with normal plasma, antibody activity identified by passive cutaneous anaphylaxis, inhibition of biological factor VIII activity, and binding to factor VIII. Despite these negative findings, it is suggested that the clinical pattern of this and previously described cases of "acquired" von Willebrand's disease has an immunologic basis that may be mediated by either humoral or cellular mechanisms.