Background and Objectives: Sentinel lymph node (SLN) biopsy is not usually performed with respect to the internal mammary lymph node chain. However, the SLN may be located in the internal mammary chain, particularly with medial lesions. We carried out this study to investigate whether lymphatic mapping and SLN biopsy can detect internal mammary involvement in patients with breast cancer. Methods: A dye-and gamma probe-guided SLN biopsy was performed in a consecutive series of 41 patients with tumor in situ or clinical stage I or II breast cancer. After the biopsy, these patients underwent either a modified radical mastectomy or breast-conserving surgery including axillary lymph node dissection. Biopsy of internal mammary lymph nodes was performed in 19 of these patients. Results: No involvement of internal mammary lymph nodes was found histologically in 5 patients in whom lymphatic flow or a "hot nodule" in the internal mammary chain was found using lymphoscintigraphy. Nodal involvement was demonstrated histologically in only 1 of 5 cases where lymphatic vessels showed dye staining or faintly stained nodes. Internal mammary lymph node biopsy also was performed in 14 of 36 patients with neither stained lymphatic vessels or nodes, nor with lymphatic flow or a hot nodule by lymphoscintigraphy. Nodal involvement was found histologically in 1 of these patients. Conclusion: SLN biopsy guided by lymphatic mapping is unreliable for identifying metastases to internal mammary lymph nodes.