The chymotrypsin-like (ChT-L) activity is one of the key regulators of intracellular protein degradation. Elevated proteasome ChT-L activity has recently been reported in plasma of patients with leukemia and myelodysplastic syndrome and suggested to have a prognostic significance. The aim of the present study was to evaluate plasma proteasome ChT-L activity in patients with newly diagnosed solid tumors at early and advanced stages of the disease. The activity was assayed using the fluorogenic peptide substrate, Suc-Leu-Leu-Val-Tyr-AMC, in a cohort of 155 patients with early/advanced rectal (n=43/29), gastric (n=6/13), and breast (n=37/27) cancer and compared with that in normal individuals (n=55). The median plasma proteasome ChT-L activity was elevated by 20-32% in patients with advanced stage of rectal, gastric, and breast cancer compared with healthy donors. The difference turned out to be statistically significant (P<0.001). By contrast, values in patients with early stage of malignancies were not significantly different from those observed in normal individuals. We also found that plasma proteasome activity correlated with serum carcinoembryonic antigen levels in the group of patients with rectal cancer (r=0.433, P<0.05). Elevated plasma proteasome ChT-L activity is indicative of advanced stage of rectal, gastric, and breast cancer. However, the activity does not seem to be a parameter with clinically relevant potential in terms of early detection of cancer in this subset of patients.