Objective We elucidated the effectiveness of a humanized CCR4 antibody (mogamulizumab) on adult T-cell leukemia-lymphoma (ATL), which typically has a poor outcome. Methods We retrospectively analyzed 14 patients with aggressive ATL who had been treated at our institution with weekly cycles of mogamulizumab for eight weeks from 2012-2014. Results The patients (median age: 63 years old) were classified as having acute- (n=10) or lymphoma-type (n=4) ATL. The prior treatment regimens consisted of CHOP, VCAP-AMP-VECP, DeVIC and CHASE, with an average of two courses (range: 1-4). The prior disease responses were partial remission (n=3) and progressive disease (n=11). The treatment was administered in the primary refractory setting (n=8), for relapse (n=2), or as bridging therapy before hematopoietic stem cell transplantation (n=4). The overall response rates were 64% and 43% after four and eight cycles (or after the final cycles), respectively. The median overall survival (OS), OS rate at six months and OS rate at 12 months were 66 days, 41.7% and 20.8%, respectively. All of the patients with acute-type ATL who showed a response to treatment had an early response. Notably, six of the 14 ATL patients showed somewhat prolonged survival (>100 days). However, relapse or disease progression in the peripheral blood, central nervous system, lymph nodes, skin, and/or bone occurred within a relatively short period after treatment. The adverse effects were tolerable, and included lymphopenia, cytomegalovirus infection and skin rash. Conclusion Mogamulizumab therapy resulted in an early and high remission rate and somewhat prolonged survival in patients with refractory ATL. However, the duration of remission was short, and there was early relapse and disease progression. This study may show the current impact of mogamulizumab in clinical practice.