To find the predictors of Gleason score upgrading in a cohort of low-risk prostate cancer patients, data were analyzed comprising 1,632 consecutive men with low-risk prostate cancer who underwent radical prostatectomy between 1993 and 2009. Assessment focused on preoperative parameters including patient age, race, diagnostic prostate-specific antigen (PSA) levels, clinical stage and biopsy Gleason score, along with pathological parameters including percentage of tumor involvement (PTI), tumor laterality, pathological stage, extra-capsular extension, seminal vesicle invasion, and surgical margins. These parameters were analyzed using univariate and multivariate methods. Kaplan–Meier curves compared differences in biochemical disease-free survival in men having cancers with and without Gleason score upgrading. Cases involving pathological Gleason score upgrading were identified in 723 (44.3 %) of 1,632 patients. Kaplan–Meier PSA recurrence-free survival curves showed a difference in outcome between men with and without Gleason score upgrading (p < 0.001). Of Gleason score upgraded patients, 35 (4.8 %) men had PTI of <5 %, 237 (32.8 %) had PTI of 5–9.9 %, 177 (24.5 %) had PTI of 10–14.9 %, and 274 (37.9 %) had PTI ≥ 15 % (p < 0.001). PTI (p < 0.001) along with diagnostic PSA, patient age, diagnostic biopsy Gleason score, pathologic stage, and surgical margin status were independent predictors of pathological Gleason score upgrading on multivariate logistic regression. PTI correlates closely with Gleason score upgrading in a low-risk prostate cancer cohort. Low-risk prostate cancer patients with clinical findings suggestive of high PTI or large volume cancers should not benefit from active surveillance strategies.