Biplane muscle-splitting (BMS) breast augmentation is a relatively recent addition to the array of mammaplasty tech-niques. Although this method has become popular since its introduction by Baxter1 and Khan,2 an anatomic basis for safely splitting the pectoralis major has not been described. We sought to delineate the surgical anatomy of the BMS pocket that confers the aesthetic benefits of this method without potentially damaging potential morbidity of the pectoral nerves injury.