An ipsilateral axillary mass was detected on pre-operative ultrasound in a 79-year-old woman with newly diagnosed breast carcioma. The mass had sonographic features that were initially thought to represent a lymph node completely replaced by metastatic disease. Though ultrasound-guided axillary fine-needle aspiration was attempted, it could not be performed due to pain. At surgery, the mass was found to be a brachial plexus tumor.