To evaluate the prognostic significance of DNA ploidy in endometrial stromal sarcoma, the traditional clinical and histopathological prognostic variables and DNA ploidy in 48 patients with histologically verified endometrial stromal sarcoma were analyzed. Evaluable flow cytometric DNA histograms from paraffin-embedded tissue from the tumor were obtained in 47 patients. In univariate analysis, malignancy grade (P < 0.001), cellular atypia (P < 0.001), tumor diameter (P = 0.001), and mitotic count (P = 0.002) were highly significant. Also menopausal status (P = 0.011), FIGO stage (P = 0.035), and free resection margins at primary surgery (P = 0.026) obtained significance, while vessel invasion and age did not. DNA ploidy was not significant. In Cox multivariate analysis, free resection margins at primary surgery were found to be the most important prognostic factor (P < 0.001), followed by malignancy grade (P = 0.002), tumor diameter (P = 0.019), and menopausal status (P = 0.019). DNA ploidy did not obtain significance. Free resection margins at primary surgery, malignancy grade, tumor diameter, and menopausal status are important prognostic factors in endometrial stromal sarcoma.