In patients with hemophilia, the development of high-responding inhibitors to factor VIII prevents adequate replacement therapy and results in increased risk of serious bleeding episodes, poor control of joint bleeding, and progressive, debilitating joint disease. Immune tolerance therapy can eradicate inhibitors, but it is not uniformly successful. Emerging data suggest that prophylaxis using activated prothrombin complex concentrates may be effective and safe in reducing the incidence of joint bleeding during immune tolerance therapy and for patients in whom immune tolerance induction fails. However, only controlled clinical trials will ultimately demonstrate whether prophylaxis can prevent joint bleeding and damage, and improve quality of life in patients with inhibitors. Copyright 2004 Wiley-Liss, Inc.