Objective: This study aimed to determine the predictive factors for patients whose International Society of Urological Pathology (ISUP) score was upgraded in radical prostatectomy (RP) pathologies with a prostate biopsy pathology of ISUP grade group 1. Materials and Methods: Among patients who underwent RP in our clinic within 10 years, 158 patients with prostate biopsy pathology of ISUP grade group 1 were examined retrospectively. Age, serum prostate-specific antigen (PSA) level, prostate biopsy ISUP grade group, number of cores taken in the prostate biopsy, number of tumor-positive cores, RP pathology ISUP grade group, and pathological stage were evaluated. Results: The mean age (± standard) of the 158 patients whose prostate biopsy pathology was ISUP grade group 1 were 64.07 (±6.6). ISUP group upgrading was detected in 47 patients (29.7%). The mean PSA value of these patients was 10.6 ng/mL (±6.9). The mean PSA value of the other 111 patients without ISUP group upgrading was 7.98 ng/mL (±4.9). The serum PSA level was significantly higher in patients with upgraded ISUP in the RP pathology (p=0.02). The percentage of tumor-positive cores in the group with ISUP group upgrading (37%) was significantly higher than that in the group without ISUP group upgrading (27%) (p=0.01). The detection rates of surgical margin positivity (42.6% vs. 18%), capsule invasion (55.3% vs. 19.8%), and seminal vesicle invasion (23.6% vs. 3.6%) were also significantly higher in the upgraded ISUP group after RP (p<0.05). Conclusion: The results of this trial suggest that active surveillance may not be an appropriate option for patients with biopsy ISUP grade group 1 with PSA level >10 ng/mL. Moreover, the presence of a higher number and percentage of tumor-positive cores constituted risks of ISUP group upgrading with concomitant poor pathological outcomes such as surgical margin positivity, capsule invasion, and seminal vesicle invasion.