Flow cytometric DNA analysis for assessing malignant potential of colorectal carcinoma was investigated by paraffin-embedded materials. Preservation time of paraffin blocks and formalin fixation time of surgical specimens within 14 days do not influence the nuclear DNA content. There was seen a good correlation between the DNA contents of paraffin-embedded and fresh materials obtained from the same surgical specimens. Using deparaffinized tumor specimens, the nuclear DNA content was measured by flow cytometry in 144 patients with primary colorectal cancer, who had been treated from 1971 to 1985. Forty-four percent of colorectal cancer were diploid and 56% were aneuploid. There was no significant correlation between ploidy pattern and clinicopathological factors. However, the patients with aneuploid tumor had a significantly worse survival than those with diploid tumor (Generalized Wilcoxon test, p less than 0.001). The patients with aneuploid seemed to have an unfavorable survival than those with diploid in the same stage, and had a significantly worse survival in each group of negative nodes, P0 and H0. It is concluded, therefore, that the nuclear DNA content of colorectal cancer may be an important prognostic factor, being independent of pathological stage.