The long-term follow-up of patients treated with extended radical mastectomy has proved that the internal mammary node (IMN) status is an important prognosticator of breast cancer. Patients with isolated IMN involvement seem to have the same outcome as those with limited axillary disease, and these patients may therefore be overstaged in the TNM system. Sentinel node biopsy (SNB) of IMNs may be an ideal staging procedure, but lymphatic mapping studies demonstrate that data from extended radical mastectomy series cannot be extrapolated to patients suitable for SNB, where the IMN involvement is <5% overall, and around 1% for IMN metastases without axillary disease. Current evidence does not allow internal mammary SNB to be recommended as a standard procedure, but as patients with IMN involvement may benefit from adjuvant systemic treatment, internal mammary SNB should be further studied in this context.