There have been few studies of DNA ploidy and cell cycle kinetics in endometrial hyperplasia. The authors studied archival cases of proliferative endometrium, simple, complex and atypical endometrial hyperplasia and well, moderately, and poorly differentiated endometrial adenocarcinoma by flow cytometry and also evaluated the significance of the degree of cytologic atypia (low versus high) in endometrial hyperplasia relative to the occurrence of carcinoma. All proliferative endometria, all types of hyperplasia and well and moderately differentiated carcinomas were diploid. Two-thirds of poorly differentiated adenocarcinomas were aneuploid. Neither S-phase fractions or proliferative fractions (S+G2M) could distinguish among the different types of hyperplasia or predict which hyperplasias were associated with carcinomas. The degree of cytologic atypia in atypical hyperplasia was not predictive of the occurrence of carcinoma. Poorly differentiated carcinomas showed significant differences in DNA ploidy, S-phase, and proliferative fractions from endometrial hyperplasia and lower grade carcinoma. These results support the concept that there are two fundamentally different types of endometrial carcinoma.