Allogeneic hematopoietic stem cell transplantation (allo-HSCT) has proven effective in adult T-cell leukemia/lymphoma (ATL) patients. To study the graft-versus-ATL (Gv-ATL) effects after allo-HSCT, we analyzed 21 ATL patients who had been treated at our hospital. Of these, 18 had acute-, 2 had lymphoma- and 1 had chronic-type ATL; at allo-HSCT, seven patients were in CR, one was in PR, five had stable disease (SD) and eight had progressive disease (PD). Disease state after allo-HSCT was CR in 14, PR in 3, SD in 1 and PD in 3 patients. Among 15 patients who survived longer than 100 days, ATL relapsed in 10 patients, skin relapsed in 9 patients and 5 had relapsed on the skin alone. After we discontinued immunosuppressant therapy in these 10 patients, 8 manifested GVHD; ATL was ameliorated to CR in 6 patients. Donor lymphocytes were infused into two patients who did not show GVHD; one obtained CR. In five patients with skin relapse alone, four patients achieved CR following the discontinuation of the immunosuppressants. Our results demonstrate that relapse of ATL after allo-HSCT tends to develop on skin, and Gv-ATL effects played a critical role in the outcome of allo-HSCT for ATL.