The F-box protein Skp2 (Fbl1) is a positive regulator of G1-S transition and promotes ubiquitin-mediated proteolysis of the cyclin-dependent kinase inhibitor p27. Its overexpression has been implicated in cell transformation and oncogenesis in both in vitro and in vivo models. In this study, we investigated its role in human prostate cancer progression. Immunohistochemical analysis was performed on formalin-fixed paraffin sections of 622 radical prostatectomy specimens, 74 prostatic intraepithelial neoplasm specimens, as well as in 4 normal prostate organ donors assembled into tissue microarrays. We found that both luminal and basal epithelial cells in normal prostate had very low Skp2 levels, but Skp2 levels and labeling frequency increased dramatically in both premalignant lesions of prostatic intraepithelial neoplasm (P = 0.0252) and in prostate cancer (P = 0.0037). The Skp2 labeling frequency in cancer was positively correlated with preoperative serum prostate-specific antigen level (P = 0.0499) and Gleason score (P = 0.0002), whereas the Skp2 index was positively correlated with extraprostatic extension (P = 0.0454), clinical stage (P = 0.0170), as well as Gleason score (P = 0.0002). Kaplan-Meier analysis revealed that a higher Skp2 labeling index (>10) was a significant predictor of shorter biochemical recurrence-free survival time after radical prostatectomy (P < 0.0363, log-rank test). An inverse correlation of Skp2 was observed with both its biochemical target p27 expression in prostate cancer (P = 0.0003) and with its putative negative regulator, the PTEN tumor suppressor protein (P = 0.0444). These data suggest that induction of Skp2 may be causally linked with decreased levels of p27 in prostate cancer and implicate PTEN in the regulation of Skp2 expression in vivo, as previous tissue culture experiments have suggested.