Endometrial cancer and hyperplasia have long been associated with diabetes. Hyperinsulinemia may have a direct mitogenic effect on the endometrium and may inhibit the effect of progestogen therapy. This case report describes the treatment of a patient with atypical endometrial hyperplasia with an insulin-sensitizing agent. A 37-year-old patient presented after failed treatment of endometrial hyperplasia with progestogen therapy. One month after initiating metformin therapy the patient's endometrial biopsy demonstrated proliferative endometrium. This patient's atypical endometrial hyperplasia regressed after the initiation of treatment with an insulin-sensitizing agent. This relatively new class of drugs may provide an adjunct to the therapy of endometrial hyperplasia.