Between 1975 and 1991, 40 patients with newly diagnosed medulloblastoma of the posterior fossa were treated at the authors' institutions. After aggressive surgical resection 39/40 patients (98%) received craniospinal radiation therapy (RT), including a local boost. A group of 29 patients was treated with adjuvant chemotherapy. The estimated overall 5-year survival and 5-year relapse-free survival (RFS) probabilities were 75% and 65%, respectively. The 5-year survival was significantly prolonged for patients treated after 1981 as compared to those treated between 1975 and 1980 (80% vs 64%, p =.02). However, multivariate analysis identified the adequate coverage of the target volume by external RT as the only significant variable (p =.0031). The extent of resection, stage according to Chang, radiation dose to the posterior fossa (<55 Gy vs ≥55 Gy) and the use of chemotherapy did not significantly influence survival and RFS. The initial site of recurrence was the posterior fossa with or without disseminated central nervous system (CNS) disease in 5 patients, the CNS without posterior fossa in 4 patients, and the bone marrow in 2 patients. An isolated frontal relapse occurred in 1 patient. We conclude that quality control of RT plays a decisive role in the long-term outcome of patients with medulloblastoma.