Sentinel lymph node biopsy (SLNB) has recently emerged as a less invasive alternative to axillary lymph node dissection (ALND) in the treatment of breast cancer. However, SLNB has a number of limitations, and we believe that alternative strategies for staging of the axilla should be explored. The hypothesis of this proposal is that the combination of preoperative high-resolution axillary ultrasound (AUS), fine needle aspiration biopsy (FNAB), and molecular analysis using real-time reverse transcription-polymerase chain reaction (RT-PCR) represents a viable, minimally invasive alternative to SLNB. We propose a prospective cohort study to rigorously assess the diagnostic accuracy of molecular analysis of AUS-FNAB specimens. Eighty female patients with pathologically confirmed, clinically node-negative invasive breast cancer who are considered to be candidates for SLNB will be eligible for enrollment. The primary endpoint of this study is to determine the feasibility of AUS-FNAB and real-time RT-PCR to predict the pathologic status of the axilla in a proof-of-principle study. In the short term, validation of this innovative strategy is likely to reduce the number of sentinel node procedures. In the long term, we believe that AUS-FNAB may ultimately replace SLNB.