<i>Introduction:</i> Upgrading of biopsy Gleason score (GS) after radical prostatectomy (RP) to GS ≧7 is common in patients with low-grade prostate cancer in biopsy. We evaluated whether a low proportion of free PSA (%fPSA) and total PSA (tPSA) predict significant upgrading after RP. <i>Patients and Methods:</i> 122 patients with biopsy GS 5 or 6 prostate cancer and a tPSA <10 ng/ml who underwent RP in our academic center were included in the study. The utility of prebiopsy %fPSA and tPSA as predictors of significant upgrading was evaluated. <i>Results:</i> Among patients undergoing RP, upgrading to GS ≧7 was found in 61 of 122 (50%) patients. A low %fPSA was a predictor of significant upgrading of tumors with biopsy GS 5 or 6 prostate cancer in patients with tPSA <10 ng/ml. <i>Conclusions:</i> Our results suggest that the selection of prostate cancer patients for active surveillance or re-biopsy may be improved by using %fPSA as a predictor of significant upgrading.