BACKGROUND Although the demonstration of leptomeningeal dissemination is the most important predictor of poor outcome in children with medulloblastoma, there is lack of consensus on the prognostic value of a positive cerebrospinal fluid (CSF) cytology (i.e., stage M1). PATIENTS AND METHODS Eighty-six pediatric medulloblastoma patients treated in Switzerland between 1972-1991 were retrospectively studied regarding the influence of M-stage on prognosis. 39 were M0, 13 M1, 15 Mx, 17 M2, and 2 M3. RESULTS Five- and 10-year overall survival rates were 76% and 54% for M0, 68% and 50% for Mx, 36% and 25% for M1, and 22% and 22% for M2-3 (P < 0.001), respectively. No significant survival differences were observed between M1 and M2-3 patients. Among 26 patients with only postoperative CSF cytologies, seven were positive. Their outcome was similar to that of six preoperatively staged M1 and significantly different from that of M0 patients (P = 0.001). In 14 patients both pre- and postoperative CSF cytology was performed. Total agreement was observed between the pre- and postoperative results (six positive and eight negative). Among the 19 M2-3 patients CSF cytology was positive in eight, negative in five, and unknown in six. CONCLUSIONS A positive CSF cytology either pre- or postoperatively predicts for a poor outcome, similar to that observed in stage M2-3 patients. A postoperative cytology is likely to be concordant with cytologic results obtained preoperatively, and seems to have the same prognostic significance. A negative cytology, however, does not exclude a more advanced stage.