Adult T cell leukemia/lymphoma(ATLL) is known to be a very poor prognosis, and intensified chemotherapy has not yet improved this prognosis. Some successful cases of allogeneic hematopoietic stem cell transplantation (allo-HSCT) from HLA-matched siblings have been reported and allo-HSCT has been suggested as improving the prognosis of ATLL. However, HLA-matched related donor is seldom available and about two-thirds of siblings of patients with ATLL are HTLV-1 carriers. Although most of ATLL patients require an unrelated donor to benefit from allo-HSCT, there is little information about the outcome of unrelated bone marrow transplantation(UBMT) for ATLL. To elucidate the feasibility and efficacy of UBMT from HTLV-1 seronegative donor, we retrospectively analyzed the data of 33 patients who were ATLL and underwent UBMT through the Japan Marrow Donor Program. Median age of patients at transplantation was 51 years(range,24–59). The conditioning regimens of 27 patients had myeloablative intensity and 6 had reduced intensity. At transplantation, 12 patients were in complete remission, 3 were partial remission and 14 were no remission. A total of 14 patients died after allo-HSCT, and 9 of those 14 patients were lost within 100 days after HSCT due to treatment related adverse events. The overall survival(OS), progression-free survival(PFS), cumulative incidences of disease progression, and nonprogression mortality at 2 years after UBMT were 49.7%, 42.6%, 24.5%, and 32.9%, respectively. Multivariate analysis demonstrated that recipient age(P=.044) and disease status at transplantation(P=.059) were independent prognostic factors for OS and PFS. These results suggest that UBMT could be considered as a treatment option for ATLL patients without an HLA-matched related donor. Further study is required to determine the indication of nonmyeloablative conditioning based on patient’s age or disease status, because myeloablative conditioning had high incidence of treatment related mortality.