Objective: To investigate the factors to predict Gleason score upgrading (GSU) of patients with prostate cancer who were evaluated by using the International Society for Urological Pathology (ISUP) 2014 Gleason grading system. Material and methods: Between January 2008 and December 2015, we retrospectively investigated patients who had undergone radical prostatectomy and followed up in the uro-oncology outpatient clinic. The pathologic specimens of the patients were evaluated based on the ISUP 2014 classification system. The patients were divided into two groups with or without upgraded Gleason scores. Factors that could be effective in predicting upgrading such as age, prostate-specific antigen (PSA), prostate volume, D'Amico risk classification, PSA density, cancer of the prostate risk assessment (CAPRA) scores, biopsy tumor percentage, body mass index, and clinical stage parameters were compared between both groups. Results: Of the 265 patients who could be evaluated and followed up regularly, Gleason score upgrades were observed in 110 (41.5%) patients. Advanced age (p= 0.009), PSA >20 ng/mL (p= 0.036), PSA density >0.35 (p=0.005), high CAPRA score (p=0.031), and high biopsy tumor percentage (p=0.009) were discovered to be correlated with Gleason score upgrade in univariate logistic regression analysis. Advanced age alone was a predictor for GSU in multivariate logistic regression analysis (p= 0.002). Five-year biochemical recurrence-free survival rate was 86% in the non-GSU group and 55% in the GSU group (p<0.001). Conclusion: GSU risk should be taken into consideration in making therapeutic decisions for older patients with prostate cancer, and precautions should be taken against development of aggressive disease.