The validity of 48 endometrial proliferative indices derived from flow cytometric deoxyribonucleic acid histograms was tested by comparison with S-phase labeling indices. There was a positive but weak correlation. Despite this correlation, there was a good relationship between the flow cytometric proliferative indices and both the menstrual phase and degree of tissue differentiation in a larger series of 162 normal, hyperplastic, and malignant endometrial specimens examined by flow cytometry alone. Flow cytometric proliferative indices may be useful for prediction of tumor behavior in non-DNA aneuploid endometrial carcinomas that are assessed as being a good prognostic type by other tumor parameters.