Rationale and Objectives. To compare the sensitivities of ultrasound guided core biopsy and fine needle aspiration (FNA) for detection of axillary lymph node metastases in patients with a current diagnosis of ipsilateral breast cancer. Materials and Methods. From December 2008 to December 2010, 105 patients with breast cancer and abnormal appearing lymph nodes in the ipsilateral axilla consented to undergo FNA of an axillary node immediately followed by core biopsy of the same node, both with ultrasound guidance. Experienced pathologists evaluated the aspirate cytology without knowledge of the core histology. Cytology and core biopsy results were compared to sentinel node excision or axillary dissection pathology. Sensitivities were compared using McNemar's test. Results. Of 70 patients with axillary node metastases, FNA was positive in 55/70 (78.6%) and core was positive in 61/70 (87.1%) (P = 0.18). The FNA and core results were discordant in 14/70 (20%) patients. Ten cases were FNA negative/core positive. Four cases were FNA positive/core negative. Conclusion. Core biopsy detected six (8.6%) more cases of metastatic lymphadenopathy than FNA but the difference in sensitivities was not statistically significant. Core biopsy should be considered if the node is clearly imaged and readily accessible. FNA is a good alternative when a smaller needle is desired due to node location or other patient factors. This trial is registered with NCT01920139.