Metformin use is associated with reduced cancer risk in several observational studies of patients with type 2 diabetes. Results from preclinical studies in endometrial cancer show that metformin reduces cellular proliferation by inhibition of the PI3K-AKT-mTOR pathway. We tested the hypothesis that metformin would reduce cellular proliferation in vivo in atypical endometrial hyperplasia and endometrial endometrioid adenocarcinoma.