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Introduction Adult T-cell leukemia/lymphoma (ATLL) is an aggressive peripheral T cell neoplasm that is resistant to conventional chemotherapy and carries a poor prognosis. The effect of mogamulizumab, an immunoglobulin (Ig) G1 monoclonal antibody targeting CCR4 for ATLL cells, was reported in a previous phase 2 study in which mogamulizumab monotherapy was evaluated in relapsed ATLL patients. The overall response rate (ORR), median progression-free survival (PFS) and median overall survival (OS) were 50%, 5.2 and 13.7 months, respectively. It was not stated whether these values were derived in the real world or in clinical practice. Here we evaluate the clinical impact of mogamulizumab treatment in CCR-4-positive aggressive ATLL patients in clinical practice. Patients and methods We retrospectively analyzed 101 CCR-4-positive ATLL patients who received at least one cycle of mogamulizumab infusion between March, 2012 and April, 2016 in 7 facilities in Miyazaki prefecture, an HTLV-1 endemic area in Southwestern Japan. The ORR, PFS, OS and adverse effects (AEs) were evaluated. We next compared OS in patients with at least one course of mogamulizumab therapy with that in historical control patients without mogamulizumab therapy. Results Of the 101 patients, 92 were evaluable for treatment response, survival and AEs. The median age was 70 years old (range; 45 to 90), and 52 patients (51%) were more than 70 years old. According to Shimoyama's criteria, 66 patients were classified as acute type, 32 as lymphoma type, and 3 as chronic type. All 3 chronic-type ATLL patients had at least one unfavorable risk factor. Of the 101 patients, 96 had refractory or relapsed ATLL when mogamulizumab treatment was started, and the prior treatments consisted of VCAP-AMP-VECP, CHOP, DeVIC or CHASE therapy, with an average of 2 courses. In the 5 remaining cases, mogamulizumab was administered as the initial therapy for ATLL. Mogamulizumab was administered as monotherapy in 87 cases (86%), and as combination therapy with other drugs in 14 cases (14%). The ORR was 37%, including a complete remission rate of 19%. The median PFS and OS were 1.8 and 4.2 months, respectively. Among the 101 patients treated with mogamulizumab, only 26 (26%) fulfilled the inclusion criteria of the phase 2 clinical study. Among patients who met those inclusion criteria, the median PFS and OS were 6.0 and 8.4 months, respectively. The use of mogamulizumab improved OS in clinical practice. The median OS of patients receiving mogamulizumab therapy was 12 months, whereas that of patients who did not receive mogamulizumab in the historical cohort was 8.4 months. Hematologic toxicity and skin rash were the most common AEs, and both were manageable Conclusion Mogamulizumab therapy showed clinically meaningful activity in ATLL patients, with an acceptable toxicity in clinical practice. Disclosures No relevant conflicts of interest to declare.