We performed allogenic bone marrow transplantation (BMT) in two adult T-cell leukemia (ATL) patients with HLA-identical siblings as donors. One patient, with acute ATL, relapsed 3 months after BMT. The other, with chronic ATL, has become free of disease over 18 months after the BMT from his human T-cell lymphotropic virus type 1 (HTLV-1)-negative sibling, and we were unable to detect HTLV-1 in the patient's peripheral blood. Based on our results and those of others, although there have been limited numbers of patients, BMT may represent the only potentially curative treatment for ATL, and the presence of graft-vs.-host disease tends to be related to good results, which suggests the possibility that graft-vs.-leukemia effects may play an important role in allogenic BMT for ATL.