Adult T-cell leukemia-lymphoma (ATL) is a mature T-cell neoplasm caused by human T-cell leukemia virus type-Ⅰ (HTLV-1) that has a poor prognosis. Median overall survival (OS) of patients with aggressive ATL treated by chemotherapy alone is about 1 year. Although allogeneic hematopoietic stem cell transplantation (allo-HSCT) has helped achieve OS improvement in aggressive ATL, transplant-related mortality (TRM) is high. To reduce TRM in allo-HSCT, prospective studies of reduced-intensity stem cell transplantation (RIST) using hematopoietic stem cells from peripheral blood, bone marrow, and umbilical cord blood have been conducted. The results suggest that RIST is feasible for elderly patients with ATL, and the existence of mild graft-versus-host disease contributes to better survival outcomes. It has been also demonstrated that Tax-specific T-cell responses might be associated with the graft-versus-ATL effect in patients with relapsed ATL who received RIST. Lack of immunity to viruses, such as cytomegalovirus, in patients who received allo-HSCT is a critical matter. Donor selection in allo-HSCT for ATL is also important, especially with respect to HTLV-1 carrier donor cell-derived ATL. An appropriate combination of allo-HSCT and molecularly targeted therapy is needed to improve OS in patients with ATL.