STUDY OBJECTIVE: To determine the effects of cranial irradiation on neuropsychological test performance evident 9 months after diagnosis. DESIGN: A companion study to a randomized clinical trial (CCG-105). SETTING: Institutions participating in Childrens Cancer Group cooperative treatment trials. PATIENTS: Seventy-four children aged 3.0 to 6.5 years with average-risk acute lymphoblastic leukemia. Children with central nervous system leukemia at the time of diagnosis, preexisting mental retardation, or Down's syndrome or for whom English was not the primary language were not eligible for study. INTERVENTIONS: Children were randomized to receive treatment with one of four systemic chemotherapy regimens and either intrathecal methotrexate sodium during induction and consolidation plus 18 Gy of cranial irradiation or intrathecal methotrexate during induction, consolidation, and maintenance as central nervous system prophylaxis. MEASUREMENT AND RESULTS: The groups were comparable with regard to chronologic age, sex, and family socioeconomic status. Children who received cranial irradiation plus intrathecal methotrexate scored significantly lower on the McCarthy Motor Scale (P < .05) and the Token Test (P < .05) than children who received intrathecal methotrexate alone. The groups did not differ significantly on the McCarthy General Cognitive Index, Developmental Test of Visual Motor Integration, or Peabody Picture Vocabulary Test-Revised. CONCLUSIONS: Findings suggest that the combined effects of cranial irradiation and intrathecal methotrexate therapy on neuropsychological performance may be evident in young children as early as 9 months after diagnosis. Follow-up assessment of these children will reveal whether these effects remain constant, intensify, or resolve.