Recent studies have demonstrated that preoperative and postoperative factors can broadly stratify patients with medulloblastoma/primitive neuroectodermal tumors (MB/PNET) into risk groups. For children with factors that suggest poor outcome after treatment with surgery and radiotherapy, the addition of chemotherapy can improve survival. Since 1983, 26 children with poor-risk posterior fossa MB/PNET have been treated at our institution with craniospinal radiation therapy and adjuvant chemotherapy. Chemotherapy consisted of vincristine during radiotherapy and eight 6-week cycles of vincristine, cis-platinum, and 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU). Twenty-five of 26 patients (96%) who have been entered on this protocol remain alive and free of disease at a median of 24 months from diagnosis (range 6 to 50 months). Twenty patients have completed all therapy and are at a median of 32 months from initial diagnosis with no evidence of disease. These patients were compared to a group of children with similar prognostic features treated at our institution between 1975 and 1983. Actuarial disease-free survival was statistically significantly better for protocol patients than for historical control subjects (p less than 0.002). This difference was most marked in patients who had received radiation therapy alone (p less than 0.0003). Actuarial 2-year disease-free survival was 96% for patients on protocol as compared to 59% for historical control patients who had been treated with radiotherapy alone. The chemotherapy given in this protocol was well tolerated. The results of this study, although preliminary, suggest that adjuvant chemotherapy is at least transiently effective in improving the rate of disease-free survival for children with poor-risk MB/PNET.