Thirty-three children under age 20 with medulloblastoma, treated between 1962 and 1976, at the University of California and the Claire Zellerback Saroni Tumor Institute of Mount Zion Hospital, San Francisco, were retrospectively studied. A relationship between dose and local control rate was suggested by an improved five-year survival in those patients receiving doses greater than 5000 rads to the posterior fossa. The posterior fossa, either alone or with the spinal cord, was the most frequent site of failure. Results of re-irradiation for failure were encouraging and no significant complications were noted. A study of the effects of craniospinal irradiation on the hematopoietic and immune system demonstrated a marked decrease in the peripheral lymphocyte population at the completion of therapy and suggested a functional impairment of the remaining lymphocytes. Other side effects of irradiation included suppression of the hypothalamic-pituitary axis and one instance of brain necrosis. Current treatment policy and proposals for future modifications are discussed.