Cytological examination of the cerebrospinal fluid (CSF) was performed in 88 children with acute lymphocytic leukemia at the time of initial diagnosis (54 cases), during (day 1-15, 17 cases) or after (day 37-58, 17 cases) the remission induction chemotherapy. Leukemic cells were found on cytological examination in 23 cases (42.6%), 5 cases (29.4%) and 2 cases (11.8%), respectively, and the incidence of central nervous system (CNS) leukemia had a tendency to decrease during the induction chemotherapy. The follow-up study of 15 cases of CNS leukemia at diagnosis revealed that the leukemic cells in CSF had decreased within 2 weeks and then disappeared after the induction chemotherapy. These findings indicate the therapeutic effect of induction chemotherapy including prednisolone on subclinical CNS leukemia at diagnosis.