Adult T cell leukemia/lymphoma (ATL) is an aggressive T cell lymphoma with a poor prognosis. Although allogeneic hematopoietic stem cell transplantation (allo-HSCT) can be a curative treatment for ATL, a significant proportion of allo-HSCT recipients suffer from relapse/progression of ATL. Here we aimed to clarify the risk factors for and outcomes after posttransplant relapse/progression. We retrospectively reviewed 76 patients with ATL who received allo-HSCT at our institute. At the time of allo-HSCT, disease status was complete response in 17 patients, partial response in 29, stable disease (SD) in 18, and progressive disease (PD) in 12. In multivariate analysis, SD/PD at allo-HSCT, lymphoma subtype, reduced-intensity conditioning regimen, and time from diagnosis to allo-HSCT were associated with risk of relapse/progression. After allo-HSCT, 26 patients had relapse/progression at a median of 66 days (range, 13-2064 days). The 2-year overall survival rate after relapse/progression was only 19%. Compared with acute-type, lymphoma-type experienced local recurrence more frequently (1/15 acute vs. 7/11 lymphoma, P < 0.01) and had a significantly longer OS after relapse/progression (median; 112 days in acute vs. 554 days in lymphoma, P < 0.01). Since the prognosis of patients with ATL who experienced relapse/progression after allo-HSCT was poor, strategies to reduce the risk of these outcomes are warranted.