Diabetic mastopathy, although reported since 1984, is a poorly recognized diabetes complication. It more frequently affects pre-menopausal women with Type 1 diabetes mellitus and microvascular complications. The pathogenesis of this condition is believed to involve mammary tissue autoimmune reaction to the accumulation of abnormal matrix proteins, caused by hyperglycemia. The lesion often simulates breast cancer; its recognition, therefore, is important to avoid unnecessary diagnostic procedures and surgical treatments. We now report a case of diabetic mastopathy which clinically simulated breast cancer in a young pre-menopausal diabetic woman who, after sonography and mammography, was suggested surgery under suspicion of breast cancer. Histopathological examination by core-biopsy ruled out malignancy.