AbstractWe observed a rare, bilateral congenital deficiency of the pectoralis major muscle in a 72‐year‐old female cadaver in our gross anatomy dissection laboratory. The outward appearance of the anterior thoracic wall, which included well‐developed breasts, revealed no obvious abnormalities. Upon dissection, the following features were observed: 1) on the left side, the sternal portion of the sternocostal head of the pectoralis major muscle was absent, the costal portion of the sternocostal head and the clavicular head were both well developed, a normal pectoralis minor was present, and the deltoid and subclavius muscles were not hypertrophied as is often the case when the pectoralis major muscle is deficient; 2) on the right side, the entire pectoralis major muscle was absent and the pectoralis minor, deltoid, and coracobrachialis muscles were infiltrated with connective tissue and fat; and 3) on both sides, the lateral pectoral nerves were absent and the medial pectoral nerves were present. The absence of the lateral pectoral nerves suggests that the deficiencies in the pectoralis major muscles are congenital malformations resulting from a developmental failure of the embryonic muscles rather than a sequel to polio or Poland's syndrome. Clin. Anat. 16:346–349, 2003. © 2003 Wiley‐Liss, Inc.