An inherited bleeding disorder caused by a reduction in the amount or function of von Willebrand factor (vWF), which is manifested through abnormal platelet function and prolonged bleeding time. The vWF factor is a blood protein which binds platelets to blood vessels when they are injured. Absence or deficiency of the factor can, therefore, lead to uncontrolled bleeding episodes. In dogs, the most common clinical signs are spontaneous bleeding from the gums or nose, blood in the urine or gastrointestinal tract, or excessive bleeding at the time of surgery.