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Hypothesis: Timing of sentinel lymph node dissection (SLND), before or after preoperative chemotherapy (PC), for breast cancer is controversial., Design: Single-institution experience with SLND before PC., Setting: Data from prospectively collected Yale-New Haven Breast Center Database., Patients: Fifty-five SLNDs were performed before PC for invasive breast cancer in clinically node-negative patients between October 1, 2003, and September 30, 2007. The results are compared with patients who underwent SLND and definitive breast and axillary surgery before chemotherapy (control group; n = 463 SLNDs)., Interventions: If sentinel nodes (SNs) were negative before PC, no axillary lymph node dissection (ALND) was performed. If SNs were positive, ALND was performed after PC at the time of definitive breast surgery., Main Outcome Measures: Sentinel node identification rate, false-negative rate, rate of positivity, and rate of residual disease in axilla., Results: Of the 55 SLNDs performed before PC, 30 (55%) had a positive SN. The SN identification rate was 100% and the clinical false-negative rate was 0%. In the control group of those with a positive SN, 55% (56 of 101 patients) had no additional positive nodes, 25% (25 of 101) had 1 to 3 positive nodes, and 20% (20 of 101) had 4 or more positive nodes. In the group with a positive SN before PC, 69% (18 of 26 patients) had no additional positive nodes after PC, 27% (7 of 26) had 1 to 3 nodes, and 4% (1 of 26) had 4 or more nodes. Among the SN-positive patients, a pathologic complete response in the breast was found in 4 of 18 patients who had a tumor-free axilla after PC., Conclusions: Sentinel lymph node dissection before PC allows accurate staging of the axilla for prognosis and treatment decisions. Despite downstaging by PC, a significant percentage of patients had residual nodal disease in the axillary dissection.