Previously, we demonstrated that inhibition of proteasomal chymotrypsin-like (CT-like) activity in human prostate cancer (PCa) PC-3 cultures and PC-3 xenografts results in accumulation of ubiquitinated proteins, followed by induction of cell death. Studies have shown that plasma CT-like proteasomal activity may be a powerful biomarker for risk stratification in hematologic malignancies. We hypothesized that circulating proteasomes could also be used to stratify risk for patients with PCa. A total of 109 patients with suspected PCa underwent prostatic biopsies were enrolled. Subjects were divided into non-cancer, low-risk PCa, and high-risk PCa groups. Three different proteasomal activity markers (CT-like, caspase-like, and trypsin-like) were measured and compared among the three groups. The proteasomal target proteins, Ub-prs, Hsp70, Bax, and P27 in plasma and prostate tissues were also evaluated. Multivariate analysis was used to assess whether CT-like activity was a predictor of PCa progression. Only proteasomal CT-like activity in the high-risk group was statistically higher than in the non-cancer group (Pâ<â0.05). The expression of Ub-prs, Hsp70, Bax, and P27 protein was decreased in both plasma and PCa tissue of high-risk patients. CT-like activity was found to be an independent predictor of high-risk PCa. Subjects with CT-like activity â¥55 had a 2.15-fold higher risk of having high-risk PCa as compared to those with a CT-like activity of <55 (Pâ=â0.021). We found CT-like activity to be an independent predictor of high-risk PCa, and as such, it may be a good candidate as a biomarker for high-risk PCa detection and stratification.