We describe a case of acquired von Willebrand's disease (vWD) associated with monoclonal gammopathy with undetermined significance (MGUS) in a 54-year-old man who was admitted with hemarthrosis and extensive thigh muscle hematoma following arthroscopic surgery and postoperative prophylaxis with low molecular weight heparin. Coagulation tests were compatible with acquired vWD: prolonged activated partial thromboplastin time (aPTT) (56.1 s), decreased levels of factor VIII coagulant activity (23%), low concentrations of von Willebrand's factor (vWF) antigen (13%), and undetectable ristocetin cofactor activity (<10%). Infusion of a vWF-containing factor VIII concentrate failed to normalize the plasma levels of vWF-related parameters. Only additional intravenous administration of immunoglobulins led to a transient normalization of ristocetin cofactor activity, vWF antigen, and factor VIII coagulant activity. While the spontaneous bleeding tendency in this case was mild, surgery and administration of prophylactic doses of low molecular weight heparin led to life-threatening bleeding.