To provide refined prognostic information from large cohorts of women with low-grade or high-grade endometrial stromal sarcoma (ESS). We performed an observational retrospective cohort analysis of women diagnosed with low-grade or high-grade ESS from the 1998-2013 National Cancer Database. Kaplan-Meier and multivariable accelerated failure time survival analyses were performed to identify prognostic factors after multiple imputation of missing data. Recursive partitioning methods were used to rank prognostic factors in high-grade ESS. Matched cohort analyses were performed to hypothesis-test effects of adjuvant treatments. We identified 2414 and 1383 women with low-grade or high-grade ESS, respectively. Women with high-grade ESS had markedly decreased survival compared to women with low-grade ESS (five-year survival (95% CI): 32.6 (30.1-35.3%) versus 90.5% (89.3-91.8%), P<0.001). Among women with high-grade ESS, median survival (95% CI) was only 19.9 (17.1-22.1) months. Increased age and tumor size were associated with decreased survival in low-grade ESS. In high-grade ESS, additional negative prognostic factors were distant or nodal metastasis, omission of lymphadenectomy, and pathologically-positive surgical margins (all P<0.001). Use of adjuvant chemotherapy (time ratio (TR) (95% CI): 1.36 (1.17-1.58), P<0.001) and radiotherapy (TR (95% CI): 1.57 (1.32-1.87), P<0.001) were associated with increased survival for high-grade ESS. The contrasting excellent versus poor prognosis of low-grade versus high-grade ESS, respectively, was confirmed. The best treatment of high-grade ESS is early and complete surgical resection including lymphadenectomy. Adjuvant chemotherapy and radiotherapy may increase survival of women with high-grade ESS.