A 46-year-old man was admitted to our hospital with swelling of a neck lymph node in June, 2002, and was diagnosed with adult T-cell leukemia/lymphoma (ATLL). As ATLL cells were detected in the peripheral blood after two courses of multi-agent chemotherapy (LSG 15), the treatment was changed to biweekly CHOP therapy. After two courses, hematological remission was achieved. Allogeneic bone marrow transplant (allo-BMT) from HTLV- negative and HLA-matched sibling donor was performed (conditioned with cyclophosphamide 60 mg/kg x 2 and total body irradiation 12 Gy). Cyclosporine A (CsA) and short-term methotrexate (MTX) were used for graft-versus-host disease prevention. Though the HTLV- provirus DNA (Southern blot) disappeared, HTLV-I provirus DNA (real-time PCR) T-cell receptor ygammachain gene rearrangement DNA (Southern blot) were detected in bone marrow after allo-BMT. MRD disappeared after the withdrawal of CsA. After the allo-BMT transplant, a graft-versus-ATLL (GVATLL) effect may be induced by abrupt discontinuation of immunosuppression.