Because of its high cost and attendant morbidity, the necessity of axillary dissection in patients with small invasive primary tumors has been questioned. Lymphatic mapping with sentinel lymph node (SLN) biopsy is an alternative to complete axillary dissection; however, researchers have excluded patients with T1A–T1B lesions. Seven hundred patients with newly diagnosed breast cancers underwent an Institutional Review Board-approved prospective trial of intraoperative lymphatic mapping using a combination of Lymphazurin and filtered technetium-labeled sulfur colloid. An SLN was defined as a blue node and/or hot node with a 10:1 ex vivo radioactivity ratio in the SLN verus non-SLNs. All SLNs were evaluated by both hematoxylin and eosin and cytokeratin immunohistochemical stains. Of the 700 patients, 665 (95.0%) were mapped successfully. One hundred ninety-six (28.0%) had T1A–T1B tumors. Forty patients (20.4%) with T1A–T1B tumors had metastases to the SLNs. We conclude that breast cancer SLN mapping is highly accurate and sensitive when combined dye techniques (radiocolloid and vital blue dye) are utilized. This technique is particularly useful in patients with small invasive primary tumors, which, despite their size, still demonstrate a significant rate of axillary metastasis. These patients should not be excluded from lymphatic mapping protocols.