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Remarkably, more than 40 years after the inception of the Gleason grading system, it remains one of the most powerful prognostic predictors in prostate cancer. Gleason's original grading system, however, has undergone significant revision over the years, first by Gleason and his colleagues, and most recently at the 2005 International Society of Urological Pathology Consensus Conference. The consensus conference and subsequent articles proposing further modifications have helped pathologists to adapt the Gleason grading system to current urologic practice in a uniform manner. The changing definitions of Gleason pattern 3 and 4 prostatic adenocarcinoma have tended to narrow the scope of pattern 3 carcinoma and widen the scope of pattern 4 carcinoma. These modifications have had an important role in improving the inter-observer reproducibility of the Gleason system. Whether these changes have a significant impact on the clinical treatment of prostate cancer remains to be seen. However, as many of these modifications are supported only by a few studies, long-term follow-up studies with clinical end points are essential to validate these recommendations.