A patient with systemic lupus erythematosus (SLE) developed acquired hemophilia A. The patient, a 24-year-old Japanese woman, was referred to our hospital because of uncontrollable bleeding following a tooth extraction. Laboratory examination revealed prolonged APTT (116 seconds), reduced factor VIII activity (2.8 %) and the presence of factor VIII inhibitor at a titer of 46.5 Bethesda units/ml. Transfusion of prothrombin complex concentrate and activated prothrombin complex concentrate followed by administration of prednisolone and cyclophosphamide successfully arrested bleeding and reduced the factor VIII inhibitor level. Acquired hemophilia A is a rare but lethal condition. Rapid diagnosis and introduction of adequate therapies are critical.