Although pectoralis major plays an important role as a prime or assistant mover for the varied range of shoulder movement, there was no significant functional loss in ordinary activities in case of congenital absence of this muscle, because of compensation of surrounding muscles. This report describes a 22-year-old soldier with unilateral congenital absence of pectoralis major, who had difficulties in specific upper extremity movement, such as throwing or climbing. There was no visible or radiological evidence of associated anomalies of ipsilateral upper extremity. Electrodiagnostic study showed normal findings except delayed latency of medial pectoral nerve supplying rudimentary pectoralis minor of the involved side. On isokinetic study of movement for both shoulders, peak torque and average power for horizontal adduction were markedly-diminished on the involved side.