Mastectomy is a commonly performed surgery for carcinoma of breast. During surgery, pectoral fascia is removed and pectoralis major muscle is laid bare. Sternalis is a rare muscle encountered in the subcutaneous plane. We examined the operative records of 1,152 patients who underwent modified radical mastectomies between 1990 and 2000. Patients who underwent conservative breast surgery or radical mastectomy were excluded. Among 1,152 patients who underwent modified radical mastectomy, eight were identified as having sternalis, a subcutaneously placed muscle oriented craniocaudally. The thickness of the muscle varied. The muscle was spared in all patients. Sternalis is a rare muscle in the subcutaneous plane. It should not be mistaken for a mass on mammography. During surgery it is important to be aware of this rare entity and identify the muscle early so that the dissection plane is appropriate. The depth at which internal mammary nodes are irradiated may also vary in the presence of the muscle. In addition, it should not be mistaken for recurrence on follow-up.