The breast is not classically included among the organs damaged by diabetic complications. The first cases of breast lesions associated with Type 1 diabetes mellitus were only described in 1984. The disease, designated as diabetic or fibrous mastopathy, is benign but may clinically simulate breast carcinoma. Its frequency is difficult to evaluate, and its pathogenesis is not yet clearly understood. We report two cases of diabetic mastopathy, together with a review of the medical literature on this subject and a description of the main characteristics of the disease. Diagnosis is based on the clinical context (premenopausal women with longstanding Type 1 diabetes mellitus who develop a hard, painless, mobile lump on one or both breasts), radiology (dense glandular tissue on mammography and marked acoustical shadowing of sound waves on sonography), and histopathology (fibrosis and perivascular and periductal lymphocytic infiltration).