Summary Background The optimum timing of sentinel-lymph-node biopsy for breast cancer patients treated with neoadjuvant chemotherapy is uncertain. The SENTINA (SENTinel NeoAdjuvant) study was designed to evaluate a specific algorithm for timing of a standardised sentinel-lymph-node biopsy procedure in patients who undergo neoadjuvant chemotherapy. Methods SENTINA is a four-arm, prospective, multicentre cohort study undertaken at 103 institutions in Germany and Austria. Women with breast cancer who were scheduled for neoadjuvant chemotherapy were enrolled into the study. Patients with clinically node-negative disease (cN0) underwent sentinel-lymph-node biopsy before neoadjuvant chemotherapy (arm A). If the sentinel node was positive (pN1), a second sentinel-lymph-node biopsy procedure was done after neoadjuvant chemotherapy (arm B). Women with clinically node-positive disease (cN+) received neoadjuvant chemotherapy. Those who converted to clinically node-negative disease after chemotherapy (ycN0; arm C) were treated with sentinel-lymph-node biopsy and axillary dissection. Only patients whose clinical nodal status remained positive (ycN1) underwent axillary dissection without sentinel-lymph-node biopsy (arm D). The primary endpoint was accuracy (false-negative rate) of sentinel-lymph-node biopsy after neoadjuvant chemotherapy for patients who converted from cN1 to ycN0 disease during neoadjuvant chemotherapy (arm C). Secondary endpoints included comparison of the detection rate of sentinel-lymph-node biopsy before and after neoadjuvant chemotherapy, and also the false-negative rate and detection rate of sentinel-lymph-node biopsy after removal of the sentinel lymph node. Analyses were done according to treatment received (per protocol). Findings Of 1737 patients who received treatment, 1022 women underwent sentinel-lymph-node biopsy before neoadjuvant chemotherapy (arms A and B), with a detection rate of 99·1% (95% CI 98·3–99·6; 1013 of 1022). In patients who converted after neoadjuvant chemotherapy from cN+ to ycN0 (arm C), the detection rate was 80·1% (95% CI 76·6–83·2; 474 of 592) and false-negative rate was 14·2% (95% CI 9·9–19·4; 32 of 226). The false-negative rate was 24·3% (17 of 70) for women who had one node removed and 18·5% (10 of 54) for those who had two sentinel nodes removed (arm C). In patients who had a second sentinel-lymph-node biopsy procedure after neoadjuvant chemotherapy (arm B), the detection rate was 60·8% (95% CI 55·6–65·9; 219 of 360) and the false-negative rate was 51·6% (95% CI 38·7–64·2; 33 of 64). Interpretation Sentinel-lymph-node biopsy is a reliable diagnostic method before neoadjuvant chemotherapy. After systemic treatment or early sentinel-lymph-node biopsy, the procedure has a lower detection rate and a higher false-negative rate compared with sentinel-lymph-node biopsy done before neoadjuvant chemotherapy. These limitations should be considered if biopsy is planned after neoadjuvant chemotherapy. Funding Brustkrebs Deutschland, German Society for Senology, German Breast Group.