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The popularization of ultrasound-guided nerve blocks in cosmetic and reconstructive breast surgery calls for better anatomical understanding of chest wall innervation. When inserting subpectoral implants, pain from pocket dissection, stretching of muscle, and release of costal attachments may be relieved by blocking the pectoral nerves in the interpectoral (IP) space.We describe the variable anatomy of the pectoral nerves in the IP space in order to define the area to be covered for sufficient blockade, based on cadaver dissections.Twenty-six fresh cadavers were dissected bilaterally. The number, location, and course of the pectoral nerves were recorded. Distances to surface landmarks (sternum, clavicle, and costae) and ultrasound landmarks (thoracoacromial artery [TAA] and pectoralis minor muscle [Pm]) were recorded.The lateral pectoral nerve and the TAA entered together into the IP space 8.9 cm (range, 8.0–12.0 cm) lateral to the midsternal line. The medial pectoral nerve (MPN) had between 1 and 4 branches that pierced the Pm, and 69% had additional branches lateral to the Pm. The muscle-piercing MPN branches were located 3.8 cm (range, 0.4–8.1 cm) and the lateral MPN branches 5.4 cm (range, 3.0–8.4 cm) from the lateral pectoral nerve. The IP course was 2.6 cm (range, 0.7–6.5 cm). All specimens were asymmetrical in location or number of MPN branches.The MPN branches that innervate the lower part of the pectoralis major muscle are asymmetrical and variable in location and length; all located in a triangular area easily defined by sonographic landmarks, lateral to the TAA.