Despite major advances in the treatment of patients with hemophilia, a serious remaining challenge was presented by the occurrence of circulating inhibitors to Factor VIII. Because of lack of information on the natural course of patients with Factor VIII inhibitors, the relative efficacy of various modes of therapy was not established. The Division of Blood Diseases and Resources decided to sponsor a clinical investigation which would evaluate populations of hemophilia patients for Factor VIII inhibitors, follow up these patients to provide information on the natural history of the inhibitor in the hemophilia patients, and make available a reference center to monitor results and attain uniformity.