A 47-year-old premenopausal woman had a swelling in right axilla which had been diagnosed as "ectopic breast tissue" with an incisional biopsy. A subcutaneous nodule appeared two years ago. The ectopic breast containing the mass was excised with axillary dissection. It was an invasive ductal carcinoma and metastasis was detected in one lymph node. She received local radiotherapy after 6 cycles of chemotherapy and has now been taking hormonotherapy. Ectopic breast tissue has potential for malignant transformation. As its carcinoma has a worse prognosis and a higher incidence of metastasis because of delayed diagnosis, prophylactic excision may be recommended.