This study was undertaken to investigate the effects of hypothyroidism on buccal mucosal bleeding time and von Willebrand factor antigen (vWf:Ag) concentrations. Hypothyroidism was induced in 8 adult dogs by administration of iodine 131. Four healthy dogs acted as controls. Measurement of plasma vWf:Ag and serum thyroxine and triiodothyronine concentrations, and buccal mucosal bleeding time were made before induction of hypothyroidism, for 23 weeks after 131I administration, and during 5 weeks of levothyroxine supplementation. No significant changes in buccal mucosal bleeding times were noted during the study. After an insignificant increase in vWf:Ag concentration in hypothyroid dogs, levothyroxine treatment was associated with a significant decrease in vWf:Ag concentration in hypothyroid dogs when compared with controls. Results of this study suggest that hypothyroidism does not induce acquired von Willebrand's disease or significant defects in primary hemostasis.