Thirty cases of colorectal carcinoma have been evaluated for proliferative activity with the monoclonal antibody Ki 67 and with flow cytometry for ploidy, DNA index and the S-phase fraction. In the series, 30% of tumours were strictly aneuploid, 23.8% tetraploid and 45% diploid: "non diploid" cases accounted for 53.8%. The mean comprehensive values of DNA index, percent S-phase and Ki 67 positive fraction were 1.4%, 11.5% and 50%, respectively. The parameters considered showed no statistically significant correlation with each other or with the common histo-pathologic parameters, such as grading and staging. The prevalent prognostic importance of ploidy compared to DNA index is emphasized, and particular attention is paid to diploid cases with a high growth fraction (S-phase fraction), which could make up an "at risk" category. Therefore, we attempted to identify subsets, within groups of the same histologic stage, with a different evolutive significance in relation to DNA content, percentage of positivity to Ki 67, S-phase fraction and ploidy. With such a multi-parametric approach, particular groups of patients at risk could be defined, which are perhaps more sensitive to specific support therapies.