A 48-year-old female discovered a mass in her left axilla. A thorough examination resulted in a diagnosis of left invasive lobular carcinoma(ILC)of the accessory mammary gland with wide ductal spread. Considering the wide ductal spread, massive resection of the left axilla mass, left lymph node dissection, and a latissimus dorsi musculocutaneous flap procedure were performed. However, histological analysis revealed ILC measuring 80×50 mm with lymph node metastases(5/23)and extensive cancer spread, resulting in a positive surgical margin. It is important to recognize the characteristics of ILC, axillary accessory breast cancer, and the axilla in a treatment strategy.