The authors have reviewed 143 cases of medulloblastoma in children aged 19 years or younger who were treated in a 42-year period and reported in the Connecticut Tumor Registry. About 20 cases have occurred in each 5-year period since 1950, but 31 were seen between 1955 and 1959. Correspondingly, an excessive number of children born in the period 1954 to 1958 have developed medulloblastomas. A relationship to polio vaccine contaminated with SV40 virus may exist. Children with medulloblastomas had an increased number of immediate family members with brain tumors, leukemia, and childhood cancer when compared to controls. In this series, the male to female ratio was 1.33:1. Average age at diagnosis was 6 1/2 years, with most children being diagnosed at 3 years old and fewer cases appearing in each successive hemidecade from birth to 20 years of age. Probability of survival at 6 months was 0.687; at 1 year, 0.444; at 2 years, 0.314; and at 5 years, 0.222. Survival probability was statistically significantly better in the years 1968 to 1977 than in previous decades, in part due to fewer autopsy diagnoses and lowered operative mortality, but also due to a decreased mortality rate in children several years after diagnosis. Fifty-one percent were treated with operation and irradiation, 17% with operation alone, 12% with irradiation alone, and 5% with operation, irradiation, and chemotherapy. Fifteen percent were not treated. One- and 5-year survival rates in patients with operation and irradiation were, respectively, 0.615 and 0.307; with operation, 0.125 and 0.042; with irradiation, 0.688 and 0.277; and with operation, irradiation, and chemotherapy, 0.857 and 0.643. All seven children who received chemotherapy were diagnosed after 1968, and five are still alive. Perhaps due to short follow-up time, the course and mortality rate of children treated with all three modalities were not statistically significantly different from those of children treated since 1968 with operation and radiation therapy.