Collins' law, known as period of risk for recurrence (PRR), has been discussed as a reliable scheme for the prediction of recurrence or cure of embryonal tumours including medulloblastomas. The PRR is defined as the age at diagnosis plus 9 months of gestation. According to Collins' law, a patient who has no clinical evidence of recurrence within this time period is considered cured. We have applied this system to 66 patients (age range from 0 to 47 years) who underwent surgery for medulloblastoma at a single center between 1975 and 1990. Three patients in whom a recurrence could not be verified died within 6 months. Eight patients (12.1%, age range from 1 to 14 years) survived the PRR without relapse and are free of risk for recurrence according to Collins' law. Of the remaining 55 patients, 35 showed a relapse within the PRR with a mean latency of 18 months (0-78); Twenty patients are alive with no recurrence as yet, but are still within the PRR Three patients had non-recurrent benign brain tumours not connected to the initial tumours with a mean latency of 10 (8-14) years after medulloblastoma surgery. Thus, all our patients fulfilled the criteria for Collins' law. In conclusion, a patient's risk for medulloblastoma recurrence is significantly higher before than after the completion of the PRR. Therefore, the PRR is a helpful prognostic parameter that provides additional information about the risk of medulloblastoma recurrence. A medulloblastoma cure cannot be presumed with certainty by Collins' law because relapses after the PRR have been reported in other studies (1.4% of the cases). Intracranial lesions detected after a long follow-up might be radiotherapeutically induced second tumours.