We have studied the factor VIII multimeric structure in four patients with acquired von Willebrand's disease associated with a circulating inhibitor to the factor VIII complex. In three of the four patients tested, the high and medium molecular weights bands were absent when assessed by sodium dodecyl sulphate-agarose electrophoresis. Plasma from the fourth patient contained all the factor VIII multimeric forms, although the high molecular weight bands were markedly decreased in concentration. Intravenous infusion of 1-deamino-(8-D-arginine)-vasopressin (DDAVP) resulted in the appearance of a full complement of multimers in the plasma of the two patients tested. This response was, however, transient and a return to pre-infusion multimeric composition occurred within 2-4 h. In the patients studied, an inhibitor to the factor VIII complex has induced an acquired variant von Willebrand's disease which transiently corrects after the infusion of DDAVP.