The purpose of this study is to evaluate whether central nervous system prophylactic treatment (CNSP) with cranial irradiation therapy (CrRT) 18 Gy and intrathecal methotrexate would decline the intelligence quotient (IQ) scores of children with acute lymphoblastic leukemia (ALL). In protocol TCL 842, children with ALL received CrRT 18 Gy in 12 fractions, and 5 concomitant doses of intrathecal methotrexate 15 mg/m2/dose with 15 mg as the maximum, for CNSP after remission achieved. The first IQ test was performed immediately after CNSP. Those who had no CNS relapse for more than 5 years after CNSP had a second IQ test. For children between 3 and 6 years old, the Stanford-Binet (S-B) IV test was used, and for older children, the Wechsler Intelligence Scale for Children-Revised (WISC-R) was used. Fourteen consecutive children at our hospital were enrolled. There were 7 boys and 7 girls. The age at diagnosis ranged from 3 to 9 years old. Two of them were in the high-risk group, eight in the intermediate-risk group, and four in the standard-risk group. The IQ scores of all patients fell within the average range. In the first IQ tests, the mean IQ score was 104.29 (range 83-124, S.D. 14.55). In the second IQ tests, the mean IQ score was 100.93 (range 85-128, S.D. 11.57). Statistically, there was no significant difference between the first and second IQ scores (paired t-test, two-tailed P = 0.4232; one-tailed P = 0.2116). Our findings suggested that CNSP used in protocol TCL 842 did not reduce IQ scores of children with ALL 5 years after CNSP.