Background: Endometrial stromal sarcomas (ESS) account for < 20% of uterine sarcomas. They usually express estrogen and progesterone receptors (ER/PR) and are considered hormone sensitive. Due to the rarity of these tumors, large clinical trials studying optimal treatment have not been possible. This study represents the largest retrospective study of ESS treated with AI. Methods: The clinicopathological variables and outcomes of patients (pts) with pathologically confirmed low grade ESS treated with AI at our institution between 1998-2020 were recorded. Results: 48 pts with ESS treated with AI were identified. They had a median age of 54 years (range 23-84) and BMI of 27 (range 20-50). 79% were white. 6 (12%), 9 (19%), 14 (29%) and 19 pts (40%) had stage 1,2,3,4 ESS, respectively. 37 (77%) were ER+/PR+; 2 (4%) ER+/PR- and 9 pts (19%) had unknown ER/PR status. All pts were postmenopausal at AI initiation. 12 pts (25%) had a synchronous cancer (5 of these had breast cancer {3 of the 5 presented post tamoxifen}). 23 pts (48%) received megestrol acetate and 25 (52%) an AI as first line hormonal manipulation. During their disease course, 35 pts (73%) received letrozole, 21 (44%) anastrozole and 19 (39.6%) exemestane. 22 pts (46%) were treated with more than one AI. 28 pts (58%) reported side-effects; arthralgia (33%) being the most common. 10 pts (21%) discontinued AI due to toxicity; 12 pts (25%) switched AI for toxicity (with improved tolerance in 67% of these pts). Among the 24 pts (50%) with measurable disease there were 2 partial responses (objective response rate of 8.3%). 1-year disease control rate (DCR) was (79%) for all pts and 58% in stage 4 disease. Median PFS for 1^st line AI was 161.6 months (95% CI 48.5 to 274.7). Conclusions: This study represents the largest study of AI use in ESS to date. We found the ORR to be more modest than previously reported. The majority of pts had prolonged stable disease with a DCR of 58% even in stage 4 disease. Pts who progress on one AI may benefit from trial of a 2^nd AI. A phase 2 study of interruption versus maintenance AI in locally advanced/metastatic ESS is currently underway (NCT03624244).