The survival period of children suffering from leukemia could be prolonged by using the new antileukemic agents. On the other hand, the frequency of central nervous system involvement as a complication of this disease was very high, i.e. 10 of 19 cases. Six showed signs of meningeal leukemia while they were in hematologic remission and the other 4 were within acute stage. Nine children were given methotrexate intrathecally as a prophylactic measure at the time when diagnosis of leukemia was confirmed, and did not suffer from this complication during follow-up to 6–13 months. From these data we conclude that the regular administration of methotrexate intrathecally to leukemic patients as a prophylactic measure, with routine spinal fluid examinations, is very useful in preventing or in reducing the central nervous system involvement.