OBJECTIVE: To examine data on the changes in the accuracy of the diagnosis of prostate cancer and of Gleason grading in the modern era. PATIENTS AND METHODS: The study comprised a pathological review within a multicentre study of patients with clinically localized prostate cancer diagnosed in the UK from 1991 to 1996 (inclusive) and treated by watchful-waiting or hormonal therapy alone. The clinical follow-up was available, histopathological appearances were reviewed and the Gleason score at diagnosis was compared with the Gleason score as analysed by a panel of genitourinary pathologists using internationally agreed criteria. In all, 1789 patients diagnosed with prostate cancer between 1991 and 1996 were reviewed, with disease-specific survival as the main outcome measure. RESULTS: In all, 133 patients (7%) were reassigned a nonmalignant diagnosis. There was a significant reassignment in the Gleason score for those with cancer, with increases of Gleason score across a wide spectrum. In multivariate analysis the revised Gleason score was a more accurate predictor of prognosis than the original score. CONCLUSION: Misdiagnosis and reassignment of Gleason score at diagnosis would have guided clinicians into large-scale changes in the management of patients. Current rates of misdiagnosis are unknown. If applicable nationally, these changes would have profound effects on the workload of prostate cancer management in the UK.