Three hypotheses are proposed to account for neurobehavioral impairments following treatment with cranial radiation therapy (CRT) and intrathecal (IT) chemotherapy: CNS treatments exert a synergistic effect (A x B), an additive effect (A + B), or a single-agent effect (A or B). Eighty-five long-term survivors of non-CNS cancers aged 6 to 16 years were classified into groups on the basis of CNS treatments: CRT-IT (n = 25), CRT-No IT (n = 11), No CRT-IT (n = 24), and No CRT-No IT (n = 25). Study I findings did not provide support for synergistic mechanisms; nonorthogonal analysis of variance showed interaction effects (CRT x IT) restricted to tactile-perceptual speed. However, main effects were significant for a single agent (CRT) across a wide range of measures. General intelligence, academic achievement, verbal knowledge and reasoning, and perceptual-motor abilities were found to be significantly lower among CRT-treated groups. Study II findings provided additional support for the role of CRT; Pearson correlations within the CRT-No IT group indicated significant negative associations between CRT dose estimates for cortical regions and perceptual-motor abilities.