We report a case of a 44-yar-old woman with occult breast cancer presenting as an axillary mass in whom sonography was able to detecct an involved internal mammary node also, thus helping to establish a diagnosis of breast cancer. The patient underwent extended radical mastectomy, including internal mammary lymphadenectomy. Microscopy of the removed specimen failed to find a primary breast cancer lesion. Metastatic cancer was seen in the palpable axillary node, another resected axillary node and a removed internal mammary node. The estrogen and progesterone receptor analysis of the axillary node were negative. Since occult breast cancer was found highly potential for metastasizing to the infraclavicular or internal mammary nodes, it is reasonable to treat such patients in the same way as those with palpable breast cancer, with adjuvant chemotherapy.