A rare case of carcinoma of accessory breast tissue is reported, which was suspected to be lymphoadenitis. A 44-year-old woman was admitted to our hospital because of a painful mass in the left axillary region. Physical examination revealed a mass 1.5cm in diameter, movable and with tenderness. Under the diagnosis of lymphoadenitis, an excisional biopsy was performed. Histological examination showed medullary carcinoma surrounded by normal mammary-gland tissue. So we performed a second operation. Wide resection of the left axillary region accompanied by resection of the biopsy scar, axillary tail of the left breast, and lymph-node dissection was done. No residual tumor was seen in the specimen, and the axillary tail was completely intact. A definite diagnosis of medullary carcinoma of accessory breast tissue with no lymph-node metastasis was made. Following radiation therapy to the axillary region, this patient is being observed periodically.