Both nuclear DNA content and S-phase fraction (SPF) can be helpful in predicting prognosis in certain malignancies. We investigated in a retrospective study the prognostic significance of nuclear DNA content and SPF as measured by flow cytometry of tumor specimens from 162 women with nonpretreated surgically staged FIGO stage I endometrial cancer using clearly defined inclusion criteria. A total of 139 (86%) cases were found to be diploid, whereas 23 (14%) were aneuploid. Ploidy showed a correlation with histologic grade, estrogen as well as progesterone receptor levels, and depth of myometrial infiltration. Univariate analysis of follow-up data showed an increased relative risk (RR) for recurrence-free survival (RFS) for grade 3 tumors (RR = 2.11, ns), for age (RR = 1.04, P = 0.023) as a continuous variable, and for SPF in diploid tumors (RR = 3.10, P = 0.035). In addition, univariate analysis of overall survival revealed similar results with a slightly increased relative risk for ploidy (RR = 1.52, ns). Multivariate analysis of RFS showed age as the only independent prognostic factor. Multivariate analysis of RFS for diploid tumors showed no independently significant factor; however, age as a continuous variable with a relative risk of 1.04 and SPF with a relative risk of 2.94 were of borderline significance. Our results suggest that abnormalities of the nuclear DNA content and SPF in this homogeneous group of patients are associated with clinical and morphological prognosticators; however, ploidy is no independent prognostic factor for RFS. For diploid tumors, SPF might be a possible independent prognostic factor.