OBJECTIVE Proliferating cell nuclear antigen immunohistochemical expression and flow cytometry techniques were used in this study to estimate the proliferation tendency and biologic aggressiveness in benign and malignant epithelial tumors of the colon and rectum. METHODS Thirty-five adenomas and 60 adenocarcinomas were studied immunohistochemically concerning proliferating cell nuclear antigen positivity in tumor cell nuclei. In addition, flow cytometry techniques were used to estimate the DNA content and percentage of tumor cells in the S-phase. RESULTS The mean proliferating cell nuclear antigen score for adenomas was 38 percent compared with a mean score of 50.4 percent for adenocarcinomas that were studied (P < 0.05). In dysplastic areas of malignantly transformed adenomas (n = 5), the highest proliferating cell nuclear antigen score (80 percent) was focally observed. Taking flow cytometry parameters into account, we found out that proliferating cell nuclear antigen can be used as an indirect indicator of the number of cells in the S-phase (SPF) but not as an independent prognostic factor. Statistical significance was found between Type III (aneuploid carcinomas) and increased proliferating cell nuclear antigen expression (proliferating cell nuclear antigen score > or = 60 percent). Furthermore, aneuploidy was especially found on cancer located on the left colon (44 percent vs. 14 percent of right colon neoplasms). Considering DNA ploidy of the above neoplasms, the aneuploid adenocarcinomas had a tendency for poorer prognosis especially if they were related to Dukes Stage C female patients. CONCLUSIONS The comparative assessment of the above parameters might be of considerable importance in the study of the proliferation activity of any form of colorectal neoplasia.