Desmopressin was administered intravenously to 68 patients with hemophilia and von Willebrand's disease of mild or moderate severity to assess the safety, reproducibility, and range of response to this new therapeutic alternative. A rise in factor VIII-von Willebrand factor levels was seen in 64 patients, and the magnitude was sufficient to provide normal hemostasis in 55 to 68 spontaneous or traumatic bleeding episodes, dental procedures, or operations. Thus, our experience shows that most patients with mild or moderate hemophilia and von Willebrand's disease can be treated effectively without plasma derivatives. Patients who had two or more infusions of desmopressin at different times had similar responses each time, and members of the same family also had similar responses after desmopressin infusions. Because this drug can be administered without significant side effects, it should have an important role in the management of patients with mild or moderate hemophilia and von Willebrand's disease.