We aimed to investigate the clinicopathologic features, immunohistochemical studies, and prognosis in patients with endometrial stromal sarcoma (ESS). Clinical information was reviewed retrospectively for cases of ESS (1985-2009). A histologic review and immunohistochemical staining for the estrogen receptor, progesterone receptor, c-Kit, CD-10, Ki-67, and m-TOR were performed. Sixty-one patients (median age, 44 y; range, 22-71) were eligible for analysis (1988 International Federation of Gynecology and Obstetrics Stage I, 43; Stage II, 2; Stage III, 11; Sage IV, 4; unstaged, 1). The median follow-up period for survivors was 73 mo. Of those, the patients who underwent an adnexectomy and a pelvic lymphadenectomy, 15% and 13%, respectively, revealed metastasis. There were 20 relapses/persistence, including 13 (65%) in the pelvis and abdomen and 7 (35%) in distant sites. Eight patients died from ESS at a median duration of 14.5 mo (range, 2-50 mo) after relapse. Five- and 10-yr cancer-specific survival (CSS) rates were 88% and 85%, respectively; and 5- and 10-yr progression-free survival rates were 69% and 57%, respectively. Stage, residual disease, and high proliferative index of Ki-67 were significant prognostic factors for both progression-free survival and CSS in a univariate analysis, in addition to mitotic index for CSS. Multivariate analysis selected only residual disease as an independent variable for progression-free survival and stage and residual disease for CSS. Our results support using clinical Stage I, no residual disease, low proliferative index of Ki-67, and estrogen receptor/progesterone receptor overexpression as potential biomarkers to select patients with ESS for fertility-preservation surgery (5 such patients were alive and free).