Acquired von Willebrand's syndrome in the course of Waldenström's disease.

: Acquired von Willebrand's syndrome with a regressive evolution is described in a 66 year old man with Waldenstrom's disease. An inhibitor electively directed against Ristocetin cofactor activity has been demonstrated, active in vitro after incubation at 37 degrees C. Serum fractionation showed that the inhibitor was independent of the monoclonal IgM and subsequent purification that it was IgG in nature. The results permit its classification as an auto-antibody.