Cathepsin D, B and L activity levels were determined in colorectal cancer and correlated with a number of biological and clinical parameters. Our studies have evidenced significant higher activity levels of these lysosomal enzymes in tumor cytosol compared to paired normal mucosa as well as an evident increase of tumor specific cathepsin D activity in Dukes' stage A tumors compared to later stages (B, C and D). Furthermore, significant higher cathepsin B and L activity levels were observed in Dukes' stage A compared to Dukes' stage D tumors while significant higher cathepsin B activity levels were observed in tumors ≤5 cm than in those >5 cm as well as in moderately differentiated tumors (G2) than in well differentiated (G1) ones. No further correlation between tumor specific cathepsin B activity levels and other parameters examined i.e., anatomical site, nodal status, DNA ploidy, and proliferation rate (S-phase fraction) or between tumor specific cathepsin D and L and all these parameters were observed. These results indicate that cathepsin D, B and L may be involved in colorectal tumor progression by acting, probably, at different levels of this process and suggest that the altered tumor specific activity of these proteinases may be of interest as independent, prognostic factor of malignant progression of this neoplastic disease.