Background: The utility of sentinel lymph node (SLN) surgery after neoadjuvant chemotherapy (NAC) in patients presenting with node-positive breast cancer has not been determined. The American College of Surgeons Oncology Group (ACOSOG) Z0171 trial was designed to evaluate SLN surgery after NAC in women presenting with node positive disease. Methods: ACOSOG Z1071 enrolled women with clinical T0-4, N1-2, M0 breast cancer receiving NAC. At the time of surgery, all patients were to undergo SLN surgery followed by axillary lymph node dissection (ALND). The primary endpoint was false negative rate (FNR) in women with cN1 disease with 2 or more SLNs reviewed. Positive SLNs were defined as metastases >0.2mm on H&E. The protocol encouraged dual tracer technique. A Bayesian study design with a non-informative prior was chosen to assess whether the probability that the SLN surgery FNR is greater than 10%. Results: From July 2009 to July 2011, 756 patients were enrolled from 136 institutions. Fifteen women were ineligible and 33 withdrew. Of 708 evaluable pts (668 cN1, 40 cN2), 643 had a SLN identified and an ALND (607 cN1, with indeterminate SLN results in 2); 52 pts (48 cN1) had no SLN identified and had ALND; 11 underwent ALND only (all cN1), and 2 pts had SLN only (both cN1). In patients with SLN and ALND, the SLN identification rate was 92.5% (92.7% in cN1, 90% in cN2). SLN correctly identified nodal status in 84% of the 695 pts [258 of pathologically node negative and 327 of pathologically node positive; cN1: 83.8% (549/655), cN2: 90.0% (36/40)]. Of the 643 pts with a SLN identified there was a complete pathologic response in 40.3% (40.3 % for cN1 and 50% for cN2). Of the pts with a positive SLN, the SLN was the only site of disease in 40%. For pts with cN1 disease with 2+ SLNs identified with residual nodal disease, the SLN FNR was 12.8%. In pts with dual tracer technique the FNR was 11.1%. There were no FN results among pts with cN2 disease with 2+ SLNs reviewed. Of the 40 pts with a false negative SLN of the 528 cN1 patients with 2+ SLNs examined, the number of positive nodes at ALND was 1 (50.0%); 2 (25%); 3 (10.0%) and 4–9 (15.0%). Conclusions: NAC resulted in eradication of lymph node disease in 40% of node positive breast cancer patients. SLN surgery after NAC in node positive breast cancer pts correctly identified nodal status in 84% of all patients and was associated with a FNR of 12.8%. The FNR of SLN is higher than the prespecified study endpoint of 10%. Further analysis of factors associated with FNR such as clinical response, histological findings and axillary ultrasound findings is warranted prior to widespread use of SLN in these patients. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr S2-1.