We report a flow cytometric study on ploidy in 117 colorectal cancers. An aneuploid cell population was found in more than 70% of adenocarcinomas. Ploidy was found to be stage-related; aneuploid tumors with DNA index greater than or equal to 1.4 were found mainly in Dukes' C and Dukes' D stages (P less than 0.02). Tumors of the caecum and of the ascending colon were more often found to be diploid than those of the other sites. There was a progressive increase in the proportion of cells in S-phase depending on whether they were from normal tissue, inflammatory mucosa or adenocarcinomas. The proportion of cells in S-phase was significantly larger in aneuploid tumors (P less than 0.001). The data presented above suggest that aneuploidy and the proportion of non-resting cells could be important prognostic factors for colorectal cancers. The latter are independent of the stage of the disease and histological differentiation.