We prospectively analyzed the tumor DNA content by flow cytometry in 100 patients who underwent a curative resection for colorectal cancer between August 1989 and May 1992 in order to evaluate the prognostic significance of DNA ploidy and the DNA index (DI). Patients with aneuploid tumors were found to have a significantly shorter disease-free survival than those with diploid tumors (P = 0.014). In addition, patients who had tumors with a DI greater than 1.6 had a significantly shorter disease-free survival than those who had tumors with a DI of less than 1.6 (P = 0.0001). After stratification by stage, this association was only seen in Dukes' stage C disease (P = 0.0065). Cox's regression analysis demonstrated that the DI (below or above 1.6) rather than DNA ploidy was an important independent predictor of disease-free survival. These results suggest that the DI rather than DNA ploidy provides us with important prognostic information in patients undergoing curative surgery for colorectal cancer.