In a prospective study, flow cytometric deoxyribonucleic acid analyses were performed on cell samples from benign and malignant tumors of the endometrium and benign endometrium. No ploidy aberrations were found in normal endometrium. Of 52 cases with adenocarcinomas, 38 (73%) were diploid and 14 (27%) aneuploid. Both the frequency and the degree of aneuploidy were correlated to histologic tumor grade but not to other prognostic variables such as International Federation of Gynecologists and Obstetricians stage, depth of myometrial invasion, or patient age. Patients with aneuploid tumors had a higher recurrence rate and shorter disease-free intervals as compared to those with diploid tumors. Similarly, death rates were higher and the median survival was shorter in the aneuploid group. Flow cytometric ploidy determination may therefore serve as an important prognostic parameter.