The objective of the HIGS is to determine host genetic factors, other than mutations within the factor VIII gene, that are associated with the development of inhibitors in severe hemophilia A and response to antigenic challenge by factor VIII. The study will involve three phases, enrolling family groups composed of brother pairs and their parents (Phase I) and siblings, a family group composed of a person with severe hemophilia and his parents (Phase II), and a group of unrelated people with severe hemophilia for the purpose of confirming associations identified in Phases I and II.