The purpose of this study was to determine the respective parts played by cerebral hemisphere irradiation, posterior fossa irradiation, and surgery in the poor late functional results often observed in children treated for medulloblastoma. To do this we compared the intellectual outcome in a series of 59 children operated on for medulloblastoma, who had received whole-brain irradiation, to that observed in a series of 37 children operated on for ependymoma of the posterior fossa, who had received radiotherapy only on the posterior fossa. Only patients who had survived for more than 2 years without recurrence were included. At the assessment 1 year after treatment, intellectual outcome was somewhat better in the ependymoma group, but the difference was not statistically significant. At the long-term follow-ups at 5 and 10 years the results remained stable in the children treated for ependymoma, around 60% having an IQ above 90, whereas the intellectual level of the children treated for medulloblastoma was seen to have deteriorated progressively: 20% had an IQ above 90 5 years after treatment and only 10% at the 10-year follow-up. This progressive degradation is most likely due to the irradiation of the cerebral hemispheres, as this prophylactic irradiation constituted the only difference between the two groups. Moreover, irradiation to the posterior fossa did not seem to affect intellectual functions, since in the group of children with ependymomas the proportion of IQs above 90 was high and remained stable over the years. Surgery was certainly responsible for some poor results. The percentage of IQs above 90 observed 1-2 years after treatment was between 70 and 80 when no postoperative complications occurred, and only between 20-40% in the presence of postoperative complications. Postoperative aggravation was in most cases related to a brain-stem lesion. These results encourage the reduction, when possible, of irradiation to the cerebral hemispheres and underline the importance of the quality of surgery.