When a malignant tumor invades the child's cerebellum, the cost of successful treatment is often significant cognitive morbidity. A review of neuropsychological outcome revealed that survivors of childhood medulloblastoma (MB) have long-term deficits in intelligence, memory, language, attention, academic skills, psychosocial function, and a compromised quality of life. These deficits varied with chronological age at tumor diagnosis and/or adjuvant treatment, type and duration of presenting symptoms, tumor extension beyond the cerebellum, a history of adjuvant radiation treatment, and time since treatment. The effects on neuropsychological outcome of other factors, such as post-surgical hydrocephalus, were less clear. To understand the interaction between two factors predictive of outcome, age at diagnosis and time since treatment, we analyzed IQ results for a new sample of 25 surgically-treated and radiated MB survivors, and found that age at diagnosis and time since treatment made separable contributions to intellectual morbidity. PIQ appeared to measure some general effects of diffuse cerebral insult because it varied with chronological age of the child at tumor diagnosis but was relatively constant in magnitude, once established. VIQ, in contrast, was somewhat less sensitive to age at diagnosis in treated MB survivors, but declined with time since treatment. These results are important for understanding the academic attainments and continuing rehabilitation needs of childhood MB survivors, because they suggest that these children progressively fail to assimilate new verbally-based knowledge at a developmentally-appropriate rate.