Background. We wanted to evaluate survival and functional morbidity following radiation treatment of brain tumors in children less than 4 years old. Procedure. Outcome was evaluated for 222 children who were less than 4years old when they were irradiated at University of Toronto Centres, 1958-1995. The status of the survivors with regard to focal neurological defects, vision, hearing, and education at last follow-up was recorded. In 23 adult survivors older than 21 years at last follow-up, information was obtained with regard to higher education, occupation, and living arrangements. Results. The overall 10-year survival rate was 40%, not significantly different than the 45% for 776 4-16-year-olds with irradiated brain tumors treated at the same institutions. Forty-five percent of the survivors had no major focal neurological, visual, or hearing defects. There were no major differences in the frequencies of these criteria or of schooling between 0-2-and 2-4-year-olds. Among adult survivors, older than 21 at last follow-up, 26% success-fully completed higher education, 31% were in full-time employment, and 37% had never been employed. For medulloblastoma, the 5-year survival rate was 61% for 30 children less than 3 years old and treated from 1975-1995. This compared favorably with recent reports of survival following primary chemotherapy with delayed or omitted radiation treatment. Summary. Radiation treatment of a young child with a brain tumor was associated with cure in 1 of every 3 patients. Unfortunately, quality of life for many survivors was not good. Only one of every 3 adult survivors was able to have a normal life-style. This shortfall was the result of focal neurological defects which were present from the time of first treatment, and of the long-term effects of radiation treatment. Conclusions. The search for less toxic treatment remains appropriate, but is experimental and researchers must recognize that there may be a trade-off between morbidity and mortality.