The cellular DNA content was measured with flow cytometry from paraffin-embedded material in 329 patients and metastatic tumors of the liver from the rectum in 11 patients. The classification of the DNA ploidy pattern is as follows: A stem cell peak with a DNA index of 0.9-1.1 is defined as DNA diploid tumor and DNA aneuploid tumor is that with a DNA index greater than or equal to 1.1. There was a good correlation of DNA indices (r = 0.997) obtained from flesh and corresponding paraffin-embedded specimens. It is concluded that accurate determination of DNA index from paraffin-embedded materials is possible in the majority of cases. DNA ploidy of primary tumor cells correlated with clinicopathological findings such as lymphatic invasion, vascular invasion, lymph node metastasis and hepatic metastasis (p less than 0.01), but did not correlate with extramural carcinoma invasion. The cumulative survival rate (Kaplan-Meier) of curatively resected rectal carcinomas was worse in DNA aneuploid than in DNA diploid tumors (p less than 0.01). These observation showed that the determination of DNA ploidy in rectal carcinomas may prove to be of prognostic value.