1. Twenty-four patients with medulloblastoma were seen at the Kings County Hospital Center from 1946 to 1966. This tumor was more frequent in children (17 patients) than in adults (7 patients). The age range was 5 months to 83 years. There was a male predominance in the series (15 males to 9 females). 2. These tumors are usually located in the cerebellum (23 out of 24 cases) and the patients presented with cerebellar symptoms (usually ataxia), as well as symptoms of increased intracranial pressure (headache and vomiting). 3. No patients survived who were untreated or treated by surgery alone. The survival for untreated children was 5 months; the average length of time of symptoms in all the children was 4 months. 4. Five out of 15 patients treated by radiation therapy and surgery are alive. When the tumor recurred after treatment, it was usually in the cerebellum. There was no difference in results with either fractionated doses of 2,500 r to 3,000 r in 4 to 9 weeks or doses of 4,000 r in 4 to 6 weeks. There was a significant difference in results in patients who had adequate treatment coverage of the skull (5 out of 7 alive) as against inadequate field arrangement (none out of 8 alive); 1 patient developed convulsions following radiation therapy and died 5 years and 6 months after treatment. 5. Modified life table study suggests a 5 year survival of 35 per cent and a 10 year survival of 26 per cent after radiation therapy.