PURPOSE: The sternalis muscle is an accessory muscle of the anterior chest wall. This is a rare anatomic variant reported in approximately 8 percent of the population, with variation among races. While several anatomic studies of the sternalis muscle exist, nothing in the literature addresses the implications of this muscle on staged breast reconstruction with implant. METHODS: We encountered two consecutive patients with a left sternalis muscle who underwent immediate staged breast reconstruction with a tissue expander. We offer a strategy for dealing with this rare, but known, anatomic variant during staged implant reconstruction. Firstly, recognizing the presence of a sternalis muscle mandates the elevation of the pectoralis major muscle and the sternalis muscle in continuity. Secondly, slight modifications must be made during the submuscular dissection to create a properly placed pocket for the tissue expander. RESULTS: While our encounters with this anatomic variant are few, our experiences with two consecutive patients illustrate that the reconstructive surgeon must be familiar with the sternalis muscle and be prepared to adapt his or her technique for tissue expander placement when faced with this accessory muscle. CONCLUSION: To date, no reports exist in the literature describing the operative implications of this anatomic variant. We offer our technique for dealing with this accessory muscle during staged implant reconstruction of the breast.