A 54-year-old woman visited our hospital with a palpable tumor in her left breast, which was diagnosed as invasive ductal carcinoma. Breast-conserving surgery was performed, in association with a sentinel lymph node (SLN) biopsy and back-up dissection of the axillary lymph nodes. One dyed axillary lymph node with high radioactivity was defined as an SLN, and intraoperative frozen-section analysis of the SLN was negative for metastasis. The final pathological diagnosis of the tumor was invasive ductal carcinoma, and one small lymph node, located in the retromammary space, just under the tumor, was positive for metastasis. The backup axillary lymph nodes were not metastatic. This patient was diagnosed false-negative by SLN biopsy, despite being positive for retroMLN metastasis. It should be recognized that retroMLNs are difficult to detect preoperatively, or intra-operatively, using dye or radiocolloid, if they are located in the post-tumoral retro-mammary space. RetroMLNs may be a pitfall in SLN biopsies.