Therapeutic use of interferon-alpha (IFN-alpha) has been associated with the development of a variety of autoimmune disorders. We describe a patient who received chronic treatment with IFN-alpha following autologous bone marrow transplantation for multiple myeloma. The patient developed an antibody inhibitor directed against factor VIII coagulant protein (FVIIIC) leading to a fatal hemorrhagic diathesis. The association between IFN-alpha therapy and autoimmune complications deserves wider recognition and further study.