The development of prophylactic therapy for acute lymphoblastic leukemia (ALL) in the central nervous system in children has resulted in an improvement of the prognosis and prolongation of patients' lives. On the other hand, late irradiation effects have increased with the improvement of the prognosis. Therefore, there is now an important need to lessen these late effects without deteriorating the prognosis. We have investigated the recurrence rate, the survival rate and the cause of death of ALL patients who were divided into a none prophylactic irradiation group and 15 Gy, 20 Gy and 24 Gy irradiation groups. The results show that the effect of prophylactic irradiation has been more effective in the 20 Gy group than that of the 24 Gy group. Furthermore, brain atrophy and leukoencephalopathy, which were investigated by X-ray CT in long-term survivors of post-prophylactic irradiation without recurrence, have been less in the 20 Gy group than in that of the 24 Gy group. This report presents the result that the most favorable dose for prophylactic irradiation for central nervous system leukemia of ALL in children is 20 Gy by a step-up method.