1551 Background: Obesity significantly increases risk of endometrial cancer (EC) through systemic metabolic effects and local tissue action, driven by estrogen and dysregulated insulin signaling. EC is most commonly diagnosed in postmenopausal women. Metformin, an antidiabetes drug, and lifestyle intervention were evaluated for effects on endometrial proliferation and serum biomarkers, in parallel with weight loss. Methods: Obese postmenopausal women were randomized into 4 groups for a 16 week intervention using a 2 (metformin 1700 mg/day vs placebo) x 2 (lifestyle intervention vs no lifestyle intervention) factorial design based on the Diabetes Prevention Program. Pre- and post-intervention endometrial biopsies were assessed for proliferation (% Ki67+). Body weight and serum markers were measured pre- and post-intervention (estrone, FSH, DHEA-S, SHBG, IGF-1, adiponectin, omentin, insulin, glucose, A1C, ALT, triglycerides, and others). Results: Of 576 women approached for the study, 29 women were randomized and 26 completed the study. Similar adherence was seen for placebo and metformin, with 88% and 94% of doses taken, respectively. Adverse events were grade 1 or 2, most commonly flatulence, headache, and diarrhea. Metformin+lifestyle group lost the most weight (-7.4%), followed by lifestyle (-5.2%), metformin only (-3.1%), and placebo (+0.1%). Endometrial proliferation was not changed. Overall proliferation was low (7.1% at baseline) with substantial variability even in this postmenopausal cohort. Biopsies produced limited endometrium, precluding evaluation of other tissue markers. Metformin significantly increased serum DHEA (p = 0.002). SHBG increased (p = 0.031) and insulin decreased with lifestyle intervention (p = 0.035) but were not statistically significant after multiple comparisons correction. Conclusions: Metformin and lifestyle intervention produced positive changes in serum markers and weight loss. While it is known that obese postmenopausal women are at increased risk for EC, improved biomarkers are needed to stratify risk and test prevention strategies, particularly at the endometrial tissue level. Clinical trial information: NCT01697566.