Adult T-cell leukemia (ATL) is an extremely difficult disease to treat. The median survival of patients with this disease has been reported to be only about 3 mo even with intensive treatment. We have treated five ATL patients (four males and one female, 42 to 72 yr of age) with total body irradiation (TBI). One patient who received 150 rad of TBI achieved complete remission and has been well without treatment for over a year. One hundred and thirty rad of TBI was administered to another patient, resulting in control of the leukocyte count at around 10,000/microliter and survival for 6 mo. Three patients received 100 rad of TBI. Only one of these patients has lived with leukemic cells for over a year without treatment. The other two patients relapsed rapidly and despite various kinds of treatment, such as a second course of TBI, extracorporeal irradiation of the blood, cytapheresis and combination chemotherapy consisting of pepleomycin, vincristine and a high dose of prednisolone, expired from pneumonia in 3 and 4 mo respectively. In all except the last two patients, thrombocytopenia and anemia developed and lasted for 2.5 wk to 3 mo and granulocytopenia occurred only in the patient who received 150 rad of TBI. Slight nausea and loss of appetite were noticed in one patient. It seems that 100 to 150 rad of TBI is effective against ATL, with acceptable side effects.