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Background Gleason grading is the strongest prognostic parameter in prostate cancer. Gleason grading is categorized as Gleason ≤6, 3 + 4, 4 + 3, 8, and 9–10, but there is variability within these subgroups. For example, Gleason 4 components may range from 5–45% in a Gleason 3 + 4 = 7 cancer. Objective To assess the clinical relevance of the fractions of Gleason patterns. Design, setting, and participants Prostatectomy specimens from 12 823 consecutive patients and of 2971 matched preoperative biopsies for which clinical data with an annual follow-up between 2005 and 2014 were available from the Martini-Klinik database. Outcome measurements and statistical analysis To evaluate the utility of quantitative grading, the fraction of Gleason 3, 4, and 5 patterns seen in biopsies and prostatectomies were recorded. Gleason grade fractions were compared with prostatectomy findings and prostate-specific antigen recurrence. Results and limitations Our data suggest a striking utility of quantitative Gleason grading. In prostatectomy specimens, there was a continuous increase of the risk of prostate-specific antigen recurrence with increasing percentage of Gleason 4 fractions with remarkably small differences in outcome at clinically important thresholds (0% vs 5%; 40% vs 60% Gleason 4), distinguishing traditionally established prognostic groups. Also, in biopsies, the quantitative Gleason scoring identified various intermediate risk groups with respect to Gleason findings in corresponding prostatectomies. Quantitative grading may also reduce the clinical impact of interobserver variability because borderline findings such as tumors with 5%, 40%, or 60% Gleason 4 fractions and very small Gleason 5 fractions (with pivotal impact on the Gleason score) are disclaimed. Conclusions Quantitative Gleason pattern data should routinely be provided in addition to Gleason score categories, both in biopsies and in prostatectomy specimens. Patient summary Gleason score is the most important prognostic parameter in prostate cancer, but prone to interobserver variation. The results of our study show that morphological aspects that define the Gleason grade in prostate cancer represent a continuum. Quantitation of Gleason patterns provides clinically relevant information beyond the traditional Gleason grading categories ≤3 + 3, 3 + 4, 4 + 3, 8, 9–10. Quantitative Gleason scoring can help to minimize variations between different pathologists and substantially aid in optimized therapy decision-making. [ABSTRACT FROM AUTHOR]