Cathepsin D content and activity were determined in matched paired sets of colorectal tumor tissue and normal mucosa and correlated with a number of biological and clinical parameters. Significantly higher cathepsin D activity was measured in tumor cytosol compared to paired normal mucosa (p < 0.02), in Dukes' stage A tumors compared to Dukes' B and C (p < 0.05), in tumors < 5 cm compared to those > 5 cm, or in tumors with a low proliferation rate compared to those with a high proliferation rate (p < 0.05). Moreover, significant differences in enzyme activity between tumor tissue and paired normal mucosa were observed in node-positive and G2 tumors (p < 0.05). No significant correlation between cathepsin D activity and other biological parameters was found. Further, no differences in cathepsin D content between tumor tissue and paired normal mucosa were observed except in Dukes' stage A tumors (p < 0.02). A significantly increased cathepsin D content was also observed in tumors > 5 cm compared to tumors < 5 cm (p < 0.01). No relationship between tumor cathepsin D content and clinical stage was detected. However, a significant correlation (p < 0.05) was observed between the tumor-specific content of this enzyme and tumor grade. Finally, there was no relationship between tumor-specific cathepsin D activity and content (r = -0.27, p = 0.23). These data suggest that cathepsin D activity rather than content correlates with the malignant progression of colorectal cancer. This phenomenon should be taken into consideration when clinical studies are undertaken to assess the potential prognostic value of proteolytic enzymes involved in tumor progression.