Diabetic mastopathy is an unusual stromal fibrotic lesion, but typically occurs in longstanding insulin dependent and younger diabetic patients. We report a case of diabetic mastopathy in an older diabetic patient. The patient was a 76-year-old woman with a history of type 2 diabetes mellitus for 13 years and 3 years of insulin treatment. She developed a 3 cm, hard, mobile nodule in the left breast. Mammograhy revealed a dense mass. Ultrasonography showed an irregular-shaped hypoechoic lesion with an unclear boundary and acoustic shadowing. Since fine needle aspiration biopsy delivered insufficient material and core needle biopsy did not yield any specific findings for diagnosis, clinically diabetic mastopathy was the prime suspect but breast cancer could be completely ruled out. Surgical excision was thus performed and diabetic mastopathy was confirmed pathologically. We report on this rare case of diabetic mastopathy in a 76 year-old type 2 diabetic patient.