Diabetic mastopathy is a rare condition that occurs in type I diabetic patients and that may mimic a breast cancer due to its intense fibrosis and inflammation. We report a 34 years old female consulting for a firm, painless and mobile retroareolar breast lump measuring 3.5 x 4 cm, that lead to the suspicion of a breast cancer. Mammography, mammary ultrasound and fine needle cytology were unspecific. A core biopsy confirmed the diagnosis of diabetic mastopathy. The patient continues under periodical assessments (Rev Méd Chile 2003; 131: 1305-08 )