In a prospective study, the DNA content of Feulgen-stained nuclei obtained from fresh samples of 211 colorectal adenocarcinomas was evaluated by means of image analysis. The DNA histogram classification took into account aneuploidy and S-phase fraction for diploid cases. No significant relationship was found between ploidy and sex, age, preoperative carcinoembryonic antigen (CEA), size of the tumor, histologic differentiation, or Dukes' stage. Aneuploidy was more frequently encountered in distal tumors. Preoperative CEA, histologic differentiation, Dukes' stage, and ploidy were individually associated with overall survival. In Dukes' A, B, and C tumors, patients with normal and elevated CEA had no significant difference in overall survival. A relationship was apparent between disease-free survival and site, histologic differentiation, Dukes' stage, and ploidy. Multivariate overall survival analysis did not reveal independent prognostic significance of ploidy when all Dukes' stages were considered. In contrast, Dukes' stage, differentiation, and ploidy were good indicators of higher risk of colorectal cancer-related death in patients undergoing curative surgery. Dukes' stage and ploidy were also indicators for recurrence. Thus, routine histopathologic characteristics should be used in combination with quantitative cytologic features for the definition of a relevant prognostic index in colorectal cancer.