Summary. We have studied four patients suffering from acquired von Willebrand's disease. All patients had a severe bleeding diathesis with recurrent life-threatening haemorrhages. Three of the patients had a monoclonal gammopathy and one of these developed multiple myeloma. In three patients tested, a plasma inhibitor to ristocetin cofactor activity was detected. In each case this was localized to the IgG fraction of plasma. In addition, VIII:C activity was found to be associated with the IgG fraction of patients’ plasma and altered mobility of VIII: C was detected on Laurell immunoelectrophoresis. Furthermore, plasma from all four patients and the IgG fraction therefrom resulted in a dissociation of normal VIII:C into two components separable by gel-filtration on Sepharose 6B. Finally, the circulating half-life of the three factor VIII activities was found to be markedly reduced in the patients with acquired von Willebrand's disease.

We conclude that in the patients studied the coagulation defect was related to the presence of a circulating inhibitor to the factor VIII complex and that this inhibitor was associated with the IgG fraction of plasma.