Severe type-3 von Willebrand's disease (vWD) was diagnosed in a young male rhesus monkey that had excessive bleeding from minor wounds. Plasma samples from the monkey had no detectable quantitative or functional von Willebrand factor (vWF), low Factor-VIII coagulant activity, and moderate prolongation of activated partial thromboplastin time. Testing of the affected monkey's extended family revealed a likely hereditary basis for the vWD, in that the sire and a paternal half-sister had markedly reduced plasma vWF concentration. Fresh whole blood was transfused to control frequent bleeding episodes throughout the monkey's life. Although vWD is the most common inherited bleeding disorder in humans and dogs, this is the first report of vWD in a nonhuman primate.