A prospective study of cellular DNA content was made by means of flow cytometry in a nonconsecutive series of 100 patients undergoing surgery for primary colorectal adenocarcinoma. DNA-aneuploidy was present in 80% of cases (80/100); 39% of these were multiclonal (31/80). There was no significant correlation between DNA-ploidy and the clinical and pathological features examined, except for the primary tumor site (right colon vs. left colon vs. rectum: P less than 0.001). After a minimum follow-up of 30 months, out of 40 patients with no local invasion and/or distant metastases, 100% (9/9) of those with DNA-diploid neoplasias showed no signs of disease relapse, vs. 55% (17/31) of the DNA-aneuploid cases (P less than 0.05). Furthermore, in 45 cases with a minimum follow-up of 30 months, overall survival was 90% in patients with DNA-diploid carcinomas and 43% in the DNA-aneuploid cases (P less than 0.05).