A 36-year-old woman was referred to our hospital because of leukocytosis in June 2000, and was admitted to our hospital and diagnosed as having adult T-cell leukemia/lymphoma (ATL; acute type). Complete remission was achieved with eight courses of CHOP therapy, but ATL relapsed and she was readmitted to our hospital in September 2001. Laboratory examination showed elevated levels of serum LDH and soluble IL-2 receptor, and hypercalcemia. CT examinations showed swelling of the abdominal lymph nodes and hepatosplenomegaly. CHOP therapy improved the symptoms, but recrudescence soon occurred. After two courses of salvage therapy which resulted in no remission, the patient received an allogeneic peripheral blood stem cell transplant (allo-PBSCT) from her HLA-matched sibling donor after preconditioning with BU + CY in January 31, 2002. Cyclosporin A (CsA) and short-term MTX were used to prevent GVHD. Bone marrow engraftment was prompt and acute GVHD was not found. Two months later, recurrence was seen in the form of subcutaneous tumors, but the tumors spontaneously disappeared following CsA withdrawal. At the time of writing, eight months after the transplant, remission has been maintained. A graft-versus-leukemia (GVL) effect may have been the curative action in this case.