CNS prophylaxis with 18 or 24 Gy cranial irradiation plus intrathecal methotrexate was given to 134 childhood acute lymphoblastic leukemia patients who had got bone marrow remission(M1) after remission induction chemotherapy from August 1979 to December 1986. The rate of initial total CNS relapse was 14.2%(19/134), the rate of isolated CNS relapse was 5.2%(7/134), and the rate of CNS relapse concomittantly combined with bone marrow relapse or testicular relapse was 9%(12/134). Male sex or older age was accociated with higher CNS relapes and the initial peripheral leukocyte count over 50,000/ul had higher relapse rate. Relapse with radiation dose of 18 Gy was somewhat lower than that with 24 Gy. Within 4 years after CNS prophylaxis occurred 89% of the total CNS relapses, 100% of the isolated CNS relapses, and 83% of the combined CNS relapses. Adjusted to exposed cases to risk of CNS relapse, the total CNS relapse rate was 11.9% during maintenance chemotherapy and 4.9% after maintenance chemotherapy.