A 62-year-old man was hospitalized in March 1990 due to upper abdominal discomfort and anorexia. The chest X-ray revealed an upper anterior mediastinal tumor, and abdominal ultrasound showed lymphadenopathy. Endoscopic examination of the stomach revealed a large, irregular ulcer, which was histologically confirmed to be malignant lymphoma by biopsy. Serum anti-HTLV-1 antibody was positive. A diagnosis of ATLL (lymphoma type) was made. The usual therapy for this disease, including the LSG4 and RCM protocols, was only transiently effective. Therefore, the patient was treated with CPT-11 (40 mg/m2) by intravenous infusion on 3 consecutive days with weekly repetition in November 1990. Complete remission was obtained after 2 weeks of treatment, and was maintained for 5 months. This case suggests that CPT-11 may be effective for the treatment of ATL.