We report a case of diabetic mastopathy in an elderly woman with type II diabetes. The patient, a 66-year-old woman with a 6-year history of diabetes mellitus, has been received an oral diabetic agent. She noticed a lump in her right breast two months previously. Mammography of the breast showed a low density mass, but there was no microcalcification or architectural distortion. Ultrasonography of the lesion revealed a hypoechoic mass with a distinct margin. Fine needle aspiration cytology was performed but sufficient tissue was not obtained. However, a cluster of small epithelial cells with nucleus swelling and increased chromatin was demonstrated. Excisional biopsy was performed and diabetic mastopathy was demonstrated histologically.