Spontaneous remission without any anti-cancer therapy in a 57-year-old woman with adult T-cell leukemia (ATL) is reported. The patient was referred to our department because of persistent cough and appearance of abnormal lymphocytes in the peripheral blood, and she was diagnosed as having chronic ATL. Eight months later, she was re-admitted because of cystitis, watery diarrhea and worsening of respiratory symptoms with an increase of ATL cells (WBC 31 x 10(9)/l with 56% ATL cells). Acute exacerbation of ATL was diagnosed. Interestingly, antibiotic therapy for the pulmonary and urinary tract infections brought about spontaneous reduction of the ATL cell count. Spontaneous remission of ATL continued for one year without chemotherapy. The role of infection as a trigger of acute exacerbation and spontaneous remission of ATL is discussed.