A 32-year-old Japanese male in his second remission of acute lymphoblastic leukemia (ALL) received a matched unrelated donor bone marrow transplant (BMT) from the Japan Marrow Donor Program. On day +83, a bone marrow examination revealed 5.2% leukemic cells. Despite the cessation of cyclosporine, leukemic cells in the bone marrow increased to 18.4% on day +91. Treatment was started with interferon (IFN)-alpha-2b 3 x 10(6) U/body s.c. daily on day +92 and leukemic cells in the bone marrow disappeared completely. The toxicity of IFN-alpha treatment included leukoencephalopathy consisting of somnolence, disorientation, short-term memory loss, lack of coordination and ataxia, myelotoxicity requiring multiple platelet transfusions and exacerbation of graft-versus-host disease (GVHD) of oral cavity, skin and lung. Because of progressive GVHD, IFN-alpha was discontinued on day +124. On day +132, a bone marrow aspirate showed 6.4% leukemic cells. The patient died of progressive ALL on day +178. IFN-alpha may be useful for the treatment of leukemic relapse following BMT, although its toxicity is marked.