We present the case of a 62-year-old Japanese woman with relapsed adult T-cell leukemia/lymphoma (ATLL) who was treated with humanized anti-CCR4 monoclonal antibody (KW-0761). Although this antibody was highly effective against refractory ATLL, 6 months after the final KW-0761 infusion, the patient complained of hypoxia due to diffuse panbronchiolitis. Physicians should remain vigilant to the possibility of such previously unreported late-onset adverse effects associated with KW-0761 therapy.