In patients with Haemophilia A, the development of inhibitor is a life-threatening complication of treatment. These patients are at high risk for dangerous bleeding as a result of this acquired resistance to human Factor VIII concentrate. Although treatment of bleeding complications has been improved with the introduction of an activated prothrombin complex preparation, therapy remains unsatisfactory. Two patients with Haemophilia A inhibitor were treated with high dose intravenous immunoglobulin in the expectation of an immunosuppressive effect. A rise in the antibody titre at the same time as the administration of factor VIII concentrate showed that this treatment was ineffective in patients with Haemophilia A inhibitor.