Collins postulated that patients with Wilms' tumor surviving for a period equal to their age at diagnosis plus 9 months were no longer at risk for a recurrence. Although this no longer holds for Wilms' tumor, it has been applied to patients with medulloblastoma. Case histories of patients who survived a period of risk designated by Collins' rule and later developed a histologically confirmed recurrent medulloblastoma are discussed. Reported cases violating Collins' rule constitute less than 2% of all medulloblastomas. The continued use of Collins' rule is advocated. Patients who have survived this period of risk, are neurologically stable, and have a negative computerized tomography (CT) head scan are exceedingly unlikely to develop a recurrent medulloblastoma.