To assess the prognostic value of DNA ploidy in colorectal cancer, compared with the histopathological findings, paraffin-embedded surgical specimens from 330 patients who underwent resection for primary adenocarcinoma were studied using a new modified method of flow cytometry. Of these specimens, 141 were DNA diploid and 189, DNA aneuploid, among which there were 3 DNA hypodiploid lesions. Of the ten variables studied in curative resection, DNA ploidy ranked fourth in prognostic significance according to the linear trend by the chi 2 test, after nodal status, grade of cellular differentiation, and degree of invasive growth, if the DNA ploidy pattern was classified into three categories. Conversely, DNA ploidy was the sixth most significant factor if DNA hypodiploidy was included in the DNA aneuploidy. The Cox multivariate analysis showed that DNA ploidy was one of the five significant factors independently determining prognosis; however, if adjustment for the modified Dukes' stage was made by the Mantel-Haenszel test, the survival difference between the diploid and aneuploid groups did not reach a statistically significant level. Thus, we conclude that from a practical point of view, DNA ploidy is not an essential factor which must be combined with histopathological variables for a better prediction of patient outcome.