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Background Half of the patient treated with neoadjuvant chemotherapy (NAC) for a large operable breast cancer has no axillary lymph node involvement at the time of surgery. Sentinel lymph node detection (SLND), performed after NAC, must select patient who should be spared of an axillary lymph node dissection (ALND). The application of SLND for staging the axilla after NAC for patient who initially had a proven axillary lymph node involvement remains controversial because of a low detection rate (DR) and a high false negative rate (FNR). Objective The aim of GANEA 2 trial was to assess the DR and the FNR of SLND after NAC in the particular case of patients with a proven axillary lymph node involvement. Patients and Method GANEA 2 was validated by scientific and ethical national boards. Inclusion criteria: FIGO stage T2-T3 infiltrating breast carcinoma, indication of NAC, surgery (radical or conservative) after NAC and signed consent form, Exclusion criteria: inflammatory cancer, local relapse, previous surgical removal of the tumour, mental disorder, pregnancy or no contraceptive method, contra-indication to NAC, NAC interrupted due to progressive disease. Design: Diagnosis and indication to plan a NAC, control of inclusion and exclusion criteria, consent form signature, axillary sonography with fine needle cytology before NAC to select patients with a proven lymph node involvement. After NAC patients underwent both SLND, with the combined technique Blue dye and radiolabeled colloid, and complementary ALND. Pathological procedure: Pathological analysis, of sentinel and non sentinel nodes, carried out according to standard methods and classified according the last American Joint Committee staging system and Sataloff classification. Studied parameters were detection rate, false negative rate and Sataloff grading on tumor and lymph nodes. We evaluated particularly the likelihood that the FNR in patients with one or more SLN examined was greater than 10%. Patients with no lymph node involvement before NAC underwent only a SLND with an ALND only in the case of SLN macro-metastasis with a rigorous follow up. They are not part of this abstract. Results From July 2010, to February 2014, 242 patients from 19 institutions were enrolled, with a proven axillary lymph node involvement before NAC. After NAC, 1/3 had metastasis free axillary lymph node (80/142). Detection rate was 83.1% (201/242). Half of the patients with a detection failure had an involved ALND. The false negative rate was 14.2% in the whole series but 24.5% in the case of only 1 SLN resected, and 8% in case of more than 1 SLN resected. In case of involved SLN, half of the patients had involved ALND. Considering the node Sataloff scoring, 18 of the 20 false negative cases were grade C or D (n=15 grade C, metastatic disease and therapeutic effect; n = 3 grade D, metastasis and no therapeutic effect). Conclusion Among patients treated by NAC for a large operable breast cancer with proven involved lymph node before NAC, who had only 1 SLN examined, the false negative rate was 24.5%. SLND with the combined technique, provides a FNR of less than 10% only in the case of 2 or more SLN resected. Citation Format: Jean-Marc Classe, Loic Campion, Severine Alran, Christine Tunon de Lara, Pierre Francois Dupre, Christelle Faure, Nicolas Paillocher, Serge Lasry, Marie Pierre Chauvet, Gilles Houvenaeghel, Marian Gutowski, Pascaline De Blaye, Charlotte Ngo, Emmanuel Barranger, Jean Luc Veraeghe, Celine Lefebvre, Jean Francois Rodier, Virginie Bordes, Helene Charitansky, Gwenael Ferron, Pierre Gimbergues. Sentinel node surgery after neoadjuvant chemotherapy in patient with axillary node involvement: The French GANEA 2 prospective multi-institutional trial [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P2-01-01.