A variant of von Willebrand's disease has been identified in which sodium dodecyl sulfate agarose electrophoresis provides evidence that the von Willebrand factor present is structurally abnormal. Rather than the repeating triplet seen in normal subjects and in patients with the IIA and IIB variants, a repeating doublet was present in the propositus. None of the bands had the same mobility as bands in normal subjects or previously described von Willebrand's disease patients. The larger multimers of von Willebrand factor were lacking both from plasma and platelets, and did not appear in the circulation after infusion of 1-deamino-[8-D-arginine]-vasopressin. There was a marked increase in the concentration of the smallest multimer in the propositus and his phenotypically normal children, indicating that this abnormality of von Willebrand factor is inherited in an autosomal-recessive manner.