The authors measured the cognitive function and physical growth of 51 children who had been treated for acute lymphoblastic leukemia with chemotherapy, cranial irradiation and intrathecal methotrexate, and who had remained disease-free for five to 12 years. A comparison group of 15 children treated for Wilms' tumor was also studied. Cognitive impairment and growth retardation were greater among the leukemia group. Of potentially greater significance, however, was the finding that female sex was the pre-eminent risk factor for central nervous system toxicity resulting from treatment. Cognitive impairment, short stature and excessive weight were all more prevalent among females than males. Approximately half the children were microcephalic, but there was no sex difference. Age at evaluation and diagnosis, as well as socio-economic status, were differentially related to outcomes for the two sexes. The authors believe the sex differences were indicative of a fundamental interaction between postnatal neural development and other biological processes.