This international study will identify genetic factors that may influence the development of inhibitory antibodies in patients with hemophilia A after treatment with factor VIII. Bleeding episodes in patients with inhibitors are often more difficult to treat. Previous research indicates that genetic factors play a role in the development of inhibitors. A better understanding of the influence of genes in this treatment complication may be helpful in predicting, treating or preventing inhibitors.

People in families in which one or more members have severe factor VIII deficiency and one or more have a history of an inhibitor may be eligible for this study. Participants fill out a form with questions about the person's relationship to other family members taking part in the study. Those with hemophilia provide a brief medical history, including hemophilia-related information, inhibitor history and the presence of other conditions such as hepatitis C and HIV. All participants have a blood sample taken for laboratory and research tests.