Abstract A 54-year-old woman with a 20-year history of generalized, severe psoriasis presented with diffuse ecchymoses, melena, and vaginal bleeding. The whole blood clotting time was 59 minutes and the plasma factor VIII concentration was less than 1 per cent of normal. Incubation of the patient’s plasma with cryoprecipitate containing one unit of factor VIII resulted in the rapid disappearance of factor VIII activity from the mixture. The major antifactor VIII activity of the patient’s plasma was associated with an immunoglobulin of the IgG class. The patient was initially treated with a combination of methotrexate, azathioprine, and cyclophosphamide. No improvement occurred, and after 6 weeks of therapy the drugs were discontinued. Eight months later, she presented with an expanding sublingual hematoma. Bleeding was controlled by the rapid administration of 10,000 units of factor VIII, and an intravenous dose of cyclophosphamide given concurrently resulted in a prompt decline in antibody titer. Factor VIII levels subsequently became normal and have remained normal during 7 months of follow-up observation.