von Willebrand's disease (vWD) is the commonest inherited bleeding disorder in man with an estimated incidence of 1 per thousand of the population. Acquired von Willebrand's disease (AvWD) is rare with less than 70 cases reported. AvWD is usually associated with autoimmune or clonal proliferation disorders and whilst the precise mechanism of acquired deficiency of von Willebrand factor (vWF) is poorly understood, the most likely candidate mechanism(s) are; antibodies inactivate or form a complex with immunologic or functional sites on vWF, or vWF multimers are selectively absorbed by malignant cells. Unlike hereditary vWD, the acquired form of the disease can be exceedingly difficult to manage. We report 4 cases of AvWD diagnosed at our centre over the past 3 yr. There was no evidence of a previous personal or family history of bleeding in any of the patients and AvWD was confirmed by laboratory testing. All 4 patients had a recognised primary medical condition known to be associated with AvWD (Waldenstrom's Macroglobulinaemia in 2 patients, hypothyroidism in 1 patient and monoclonal gammopathy of unknown significance (MGUS) in 1 patient). The acquired haemostatic defect corrected following treatment of the primary condition in 3 patients with the other patient requiring on demand von Willebrand Factor replacement to control spontaneous and surgery induced bleeding.