Adult T cell lymphoma-leukemia (ATL) is a highly aggressive disease and allogeneic hematopoietic transplantation (allo-HSCT) is the only therapeutic option for achieving a cure. However, some ATL patients cannot undergo HSCT. One of the important reasons for restricting HSCT in ATL is the high incidence of pulmonary complications associated with ATL including opportunistic infections, infiltration of ATL cells, and HTLV-1 associated bronchopneumonopathy. Herein, we report an ATL case with pulmonary infiltration of ATL cells successfully treated with allo-HSCT after improvement of pulmonary function with administration of the anti-CCR4 antibody mogamulizumab. To our knowledge, this is the first ATL case showing improvement of pulmonary invasion of ATL cells after treatment with mogamulizumab. In addition, this case suggests that mogamulizumab treatment might be useful as a bridge to allo-HSCT in ATL patients.