A 28-year-old woman underwent mammary lymphoscintigraphy for evaluation of the sentinel node status of a small infiltrating ductal carcinoma in the central, lower, and outer quadrant of the left breast confirmed by biopsy. The patient was scheduled for lumpectomy and sentinel node biopsy. Lymphoscintigraphy showed increased uptake at two sites of the peritumorally injected tracer (60 MBq; 1.6 mCi rhenium sulfur nanocolloid) in internal mammary nodes, two of which were located most proximal to the tumor and related to the tumor by two distinct lymphatic ducts. An axillary lymph node was also detected. At surgery, the gamma probe confirmed the location of all the hot spots, and four nodes were harvested: one axillary, two first internal mammary nodes in the third intercostal space, and one internal mammary node in the second intercostal space. Histologic analysis revealed that the axillary sentinel node was free of disease, but the two first intramuscular sentinel nodes were metastatic. The second-echelon internal mammary node above the sentinel nodes (second intercostal space) was free of disease. This case shows the benefit of imaging all the drainage pathways of a breast tumor and retrieving nonaxillary lymph nodes, which may be the only positive nodes. As a result of the findings, the patient's prognosis may be altered and the subsequent therapeutic approach may be modified. Although exclusive involvement of the internal mammary chain is well established and relatively rare (occurring in approximately 5% of cases), it is important to remember that the axillary status of a breast tumor is not the only major prognostic factor.