We have studied the roles of von Willebrand factor (vWF) and factor VIII in arterial thrombosis in four canine phenotypes: normal (n = 6), hemophilia A (n = 11), von Willebrand disease (vWD) (n = 9), and hemophilia A/vWD (n = 1). vWF activity was determined by botrocetin- induced agglutination of fixed human platelets and vWF antigen (vWF:Ag) by Laurell electroimmunoassay and crossed immunoelectrophoresis. Plasma from normal dogs and those with hemophilia A had vWF activity, vWF:Ag, and a full range of vWF:Ag multimers on gel electrophoresis equivalent to normal canine plasma pool. Platelet cytosol contents were isolated by freezing and thawing, triton X-100 solubilization, or sonication of washed platelets with and without protease inhibitors and inhibitors of platelet activation. Washed platelets were also stimulated with calcium ionophore and MgCl2. There was no measurable vWF activity or vWF:Ag in platelet lysates or releasates in any dog regardless of phenotype. All dogs were studied using a standard arterial stenosis and injury procedure to induce arterial thrombosis. Thromboses were detected by cyclic reductions in Doppler blood flow velocity. Vessels were examined by light and scanning electron microscopy. Thrombosis developed in the arteries of normal (9 of 10) and hemophilia A dogs (16 of 16) but in none of the vWD dogs (0 of 10). Infusion of canine vWF cryoprecipitate into vWD dogs markedly shortened bleeding time but did not support thrombosis as seen in dogs with vWF in the plasma and subendothelium. Thrombosis, then, fails to occur when vWF is absent from the plasma and subendothelial compartments or present only in the plasma compartment. These data are consistent with the hypothesis that vWF in the plasma and subendothelium supports thrombosis. Neither plasma FVIII nor platelet vWF is essential for thrombosis in this model.