Purpose: Allogeneic hematopoietic stem-cell transplantation (allo-HSCT) is one important treatment option for patients with aggressive adult T-cell leukemia/lymphoma (ATLL). Mogamulizumab (anti-CCR4 monoclonal antibody; Mog) was recently approved as a treatment for ATLL in Japan. Major concerns exist about the possible adverse effects of pretransplantation Mog because Mog depletes regulatory T cells for several months. We assessed the impact of pretransplantation Mog on clinical outcomes after allo-HSCT. Patients and Methods: We included 996 allo-HSCT recipients age 70 years or younger with aggressive ATLL who were given the diagnosis between 2000 and 2013 and who received intensive chemotherapy by multiple chemotherapeutic drugs as first-line therapy. Before allo-HSCT, 82 patients received Mog with a median interval of 45 days from the last Mog to allo-HSCT. Results: Pretransplantation Mog was associated with an increased risk of grade 3 to 4 acute graft-versus-host disease (GVHD; relative risk, 1.80; P < .01) and refractoriness to systemic corticosteroid for acute GVHD (relative risk, 2.09; P < .01). One-year cumulative incidence of nonrelapse mortality was significantly higher in patients with pretransplantation Mog compared with those without (43.7% v 25.1%; P < .01). The probability of 1-year overall survival was also significantly inferior in patients with pretransplantation Mog compared with those without (32.3% v 49.4%; P < .01). In particular, use of Mog with intervals < 50 days to allo-HSCT was associated with a dismal clinical outcome. Conclusion: Pretransplantation Mog was significantly associated with an increased risk of GVHD-related mortality, which supports the relevance of CCR4-expressing Tregs after allo-HSCT in humans. In clinical practice, Mog should be cautiously used for patients with ATLL who are eligible for allo-HSCT.