The present study investigated the concordance between Gleason scores assigned to prostate biopsy specimens by outside pathologists and a urological pathology expert, and determined the risk of upgrading between opinion-matched Gleason grade group (GGG) 1 biopsy specimens and radical prostatectomy specimens. Between January 2012 and May 2018, 733 patients underwent robot-assisted radical prostatectomy. Patients whose original biopsy specimens from outside hospitals were reviewed by a urological pathology expert Okayama University Hospital were included. Patients who had received neoadjuvant hormonal therapy were excluded. Logistic regression analysis was used to identify predictors of upgrading among GGG 1 diagnoses. A total of 403 patients were included in the present study. Agreement in GGG between initial and second-opinion diagnoses was present in 256 cases (63.5%). Although opinion-matched cases improved concordance between biopsy and prostatectomy specimen GGG compared with single-opinion cases (initial, 35.2%; second-opinion, 36.5%; matched, 41.4%), 71% (56/79) of cases classified as GGG 1 were upgraded after prostatectomy. Multivariate analysis revealed that prostate-specific antigen density and Prostate Imaging Reporting and Data System version 2 score were significant predictors of upgrading (odds ratio, 1.10; P=0.01; and odds ratio, 1.88; P=0.03, respectively). In conclusion, the GGG concordance rate between needle-core biopsy and radical prostatectomy specimens was higher in opinion-matched cases; however, 71% of opinion-matched GGG1 cases were upgraded after robot-assisted radical prostatectomy. Urologists should propose treatment strategies or further biopsy rather than active surveillance for patients with GGG1 and a high PSAD and/or PI-RADS score.