Methods: Classical and modified Gleason grades and scores were assigned to cases of prostate carcinoma accessioned by the Trans-Tasman Radiation Oncology Group RADAR trial. Separate scores were derived for each grading system based on the percentage of each Gleason grade per case (area-based score) and the score of the highest scoring core. The predictive performance of each of the four Gleason scores assigned to each case was evaluated using nadir prostate specific antigen (nPSA) as a clinical end point. Results: Modified Gleason scoring resulted in an upward shift of scores, primarily resulting from the reclassification of classical pattern 3 to modified pattern 4. On re-grading classical Gleason score 7 cores, there was a 64% decrease in the number of cores with < 25% Gleason pattern 4 tumour, while the number of cores with 75-100% Gleason pattern 4 tumour increased by 96%. All four scoring models performed reasonably well as predictors of nPSA; however, on comparison of the prognostic gradients of the grade groupings, classical Gleason scoring outperformed modified Gleason scoring. Conclusion: The overlap of the predictive performance of Gleason pattern 3 with Gleason pattern 4, suggests that review of the defining features of modified pattern 4 may improve the prognostic prediction of modified Gleason scoring.