Ectopic breast tissue, that includes supernumerary breasts and aberrant breast tissue, develops along the mammary line. Malignancies rarely develop in this tissue. We report a 44 years old female subjected to a resection of a 3 cm Ø tumor located 2 cm below the right lower mammary sulcus. The pathological study reported an infiltrating ductal carcinoma. The patient was operated again and a metastatic carcinoma was detected in three resected axillary lymph nodes. Adjuvant chemotherapy and radiotherapy was indicated (Rev Méd Chile 2001; 129: 663-5)