It is controversial whether cell DNA measurement is a reliable method to predict prognosis in radically resected large bowel carcinomas. A study using strict rules was undertaken on 54 consecutive patients to clarify the usefulness of DNA ploidy by image cytometric analysis as a prognostic indicator. The tumors were 39% diploid and 61% aneuploid. DNA ploidy was not related to more advanced stages and it, with histological grade and Dukes' stage, was an independent covariate and influenced both disease-free and overall survival. Aneuploid tumors showed the worse prognosis in all Dukes' stages. This prospective study shows that DNA ploidy is an important independent prognostic indicator of clinical outcome and recurrence rate, and it should be used to both select high-risk patients and guide postoperative treatments, particularly in early-stage large bowel carcinomas.