Fifty-eight consecutive patients undergoing a modified radical mastectomy were subjected to complete dissection and pathological assessment of the interpectoral fascia and the group of lymph nodes it contains. The dissection was camed out in all patients, irrespective of whether they were palpable or not. Interpectoral nodes (IPNs) were anatomically present in 28 patients (48%) and were completely absent in 30 patients (52%). Ten patients were Stage I, 18 were Stage 11, and 30 were Stage 111. Of the 25% (15/58) of patients with microscopic metastasis, only 12/15 had palpable nodes; 66% (1O/15) of patients had axillary and apical nodes positive. Significantly, two patients with negative nodes in the axillary and apical group had metastatic Rotter's nodes. Of the 15 patients with positive IPNs, nine had primary tumors located within the upper quadrants of the breast, whereas only five had tumors in lower quadrants and one had a centrally located tumor. The neurovascular bundle to the pectoralis major could be safely preserved in 93% (54/58) of patients. The incidence of impalpable nodes with microscopic metastasis and the evidence of exclusively metastatic interpectoral nodes with uninvolved axillary and apical nodes prompt the following conclusions: (1) interpectoral fascia and nodes should be mandatorily dissected in all patients irrespective of the nodes being palpable or not; (2) the dissection is anatomic and is associated with almost no additional morbidity; ( 3 ) the group of patients with IPNs positive and the axillary group negative, would benefit maximally from the IPN dissection. Similarly, this dissection in all other groups of patients would enable a more accurate staging and selection of therapeutic strategies.