Fresh tissue specimens from 406 colorectal carcinomas were analyzed by DNA flow cytometric study, and the DNA ploidy pattern was compared with Dukes' stage, histologic grade, and degree of cellular atypia. Sixty-one percent of the carcinomas had a distinct aneuploid DNA pattern. The proportion of aneuploid carcinomas was significantly higher in the advanced Dukes' stages than in the localized ones. A highly significant association was found between DNA ploidy pattern and degree of cellular atypia, whereas no association was demonstrated between DNA ploidy pattern and histologic grade. This finding might indicate that cellular atypia has a stronger prognostic impact than the growth pattern of the tumor. The authors suggest that flow cytometric DNA quantification may replace assessment of cellular atypia in the histologic evaluation. Furthermore, together with earlier findings by others, these results indicate that the degree of cellular atypia may be conserved during the development from adenomas to carcinomas.