Nine long-term survivors of cerebellar medulloblastoma treated with surgery and irradiation were retrospectively examined with a complete battery of neuropsychological tests and the results compared with their nonirradiated siblings. Significant decreased scores were found in the full-scale intelligence quotients (IQ) (P less than 0.001), performance IQ (P less than 0.001), and verbal IQ (P less than 0.02) with all nine irradiated patients scoring below their siblings. Also, educational quotients (EQ) of the irradiated patients were 12 to 17 points below the nonirradiated siblings with arithmetic EQ significantly decreased (P less than 0.01). Most severely affected were those children younger than 8 years at time of irradiation. No correlation was found with whole-brain dose, or objective physical or neurologic findings.