Significance of flow cytometric DNA analysis for assessing malignant potential and survival of colorectal cancer was investigated using paraffin-embedded materials from 144 patients with primary colorectal cancer who had been treated from 1971 to 1985. Forty-four percent of colorectal cancer were composed of diploid and 56 percent were aneuploid. DNA indices (DI) of aneuploid tumors showed a bimodal distribution. There was no significant correlation between ploidy pattern and clinicopathological factors. While, DI level showed significantly higher in poorly differentiated adenocarcinomas and in clinicopathological stage III and V tumors. Overall survival in the patients with aneuploid tumor was significantly worse than that in those with diploid tumor (p less than 0.001). Survival rate was poorer in the patients with aneuploid tumor than in those with diploid tumor, who were stratified according to categories of curable resection, stage, histological type, negative peritoneal or hepatic involvement and negative node metastases. However, there was no significant relation between DI and survival among the patients with aneuploid tumor. From these results, it was concluded that the nuclear DNA content of colorectal cancer may represent biological malignant potential of the disease, and that the DNA ploidy pattern may be an important prognostic indicator, being independent of clinicopathological factors.