Our objectives were 1) to compare the efficacy of progestin therapy combined with metformin (Prog-Met) to Prog alone as primary fertility sparing treatment in women with atypical hyperplasia/endometrial intraepithelial neoplasia (AH/EIN) or early-stage endometrioid carcinoma (EC), and 2) to analyze the proportion of women achieving live birth following treatment. A retrospective cohort study of all reproductive-aged women with AH/IN or EC treated with Prog ± Met from 1999-2018 was conducted. Complete response (CR) was assessed and Kaplan-Meier analysis used to calculate time to CR. Comparison of potential response predictors was performed with multivariable Cox regression models. Ninety-two women met criteria; 59% (n = 54) were treated for AH/EIN and 41% (n = 38) for EC. Their median age, body mass index, and follow up time was 35 years, 37.7 kg/m Our study does not support the use of Prog-Met therapy for treatment of AH/EIN or EC. Additionally, fewer than 20% of women achieved a live-birth pregnancy during the study period, with most requiring ART.