Extensive axillary lymph node involvement in breast cancer patients implies poor prognosis, and is an indication for chest wall irradiation. Patients presenting with small tumors and a negative axillary status clinically, are expected to have a good prognosis. A treatment strategy chosen based on such assumption might prove to be sub-optimal in case of extensive axillary involvement. Our goal was to determine the incidence of extensive axillary involvement of four nodes or more in patients with T1-T2 tumors, and to evaluate the potential consequences of pre-operatively underestimated extensive axillary disease. We reviewed the charts of patients who underwent sentinel lymph node biopsy for primary T1-T2 invasive breast cancer, with a negative pre-operative axillary assessment. Tumor size, histology, and rates and extent of axillary involvement were noted. Of 239 patients, 71 (29.7%) had involved axillary nodes. Fifty-eight of these 71 patients had 1-3 involved nodes and the remaining 13 patients had 4 to 18 involved nodes. Of 168 patients with T1 tumors, 3 (1.8%) had 4 to14 metastatic nodes, and of 71 patients with T2 tumors, 10 (14%) had 4-18 metastatic nodes. A small percentage of patients undergoing sentinel lymph node biopsy have advanced loco-regional disease due to significant axillary nodal involvement. This should be taken into consideration when planning immediate reconstruction. Prophylactic measures such as contra-lateral mastectomy in patients at high risk for a second primary tumor, when considered, might better be deferred until the final pathology report is available. This is so patients with poor prognosis, evident by extensive lymph node involvement not known pre-operatively, do not undergo un-necessary prophylactic surgery such as contra-lateral mastectomy they will probably not benefit from.