An operative method of extended radical mastectomy involving intrapleural en bloc resection of the internal mammary lymph nodes by sternal split was proposed. The operation enables complete resection of axillary and internal mammary lymph nodes. Seventy patients with primary breast cancer underwent the extended operation and two patients with recurrence in the internal mammary lymphnode chain following standard radical mastectomy underwent resection of internal mammary lymphnodes by sternal split, with no fatalities and no increase in postoperative disabilities. Metastases to the internal mammary lymphnode chain were histopathologically found in 14 of 70 patients with primary breast cancer and in two with recurrence in the internal mammary lymphnode chain, following standard radical mastectomy; The location of the metastatic internal mammary lymphnodes was from just below the subclavicular vein to the third intercostal space along the internal mammary vessels. Cancer cells were seen not only in the lymphnodes, but also in lymphatics of areolar tissue near the node or in lymphatics between the parietal pleura and endothoracic fascia in patients with primary breast cancer. And cancer invasion to parietal pleura was seen in patients with recurrence in the internal mammary lymphnode chain following standard radical mastectomy. All these findings indicate the rationality of our extended procedures.