Autoantibodies towards coagulation factor VIII is a rare disease, incidence 1 pr. 2.5-5 million/year. The symptoms are most often subcutaneous or intramuscular haemorrhages or uncontrollable bleeding after minimal traumas. Screening tests show prolonged activated partial thromboplastin time, normal prothrombin time and thrombocyte count. Production of autoantibodies is controlled by prednisolone which may be supplemented with chemotherapy, i.e. azathioprine. Bleeding can be controlled by using coagulation factor concentrates that bypass factor VIII. If diagnosed early, there is a good chance of both stopping bleeding and suppressing autoantibody production. In order to be able to detect patients at risk of having factor VIII autoantibodies, it is recommended to screen all bleeding patients using activated partial thromboplastin time, prothrombin time and thrombocyte count. All patients showing isolated prolonged activated partial thrombin time should be referred to a laboratory specialized in coagulation problems for immediate evaluation.