One hundred patients who underwent surgery for large bowel carcinoma between 1978 and 1982 were examined by flow cytometric DNA quantitation of fresh tumor specimens and divided into an aneuploid (AN) group of 63 and a near diploid (ND) one of 37. All patients were followed until death (n = 63) or until December 31, 1988. Forty-one patients (65%) with AN tumors died of cancer, as did 12 patients (32%) with ND carcinomas. Thus patients with ND tumors had a better survival rate (P = 0.04) than did those with AN ones. The difference was apparent in Dukes' Stages A, B, and C, but not in Stage D. All patients with tumors in this stage died from their carcinomas irrespective of ploidy group. Multiple regression analyses (Cox) of prognostic factors revealed that the most important prognostic variables were (in descending order) Dukes' Stage D, Dukes' Stage C, and DNA ploidy pattern. Histologic grade was not significant as an independent prognostic variable. These results indicate that the presence of a distinctly aneuploid DNA ploidy pattern in large bowel carcinoma is an important prognostic variable that worsens survival rates significantly.