In order to clarify the cause of the atrophy of the pectoral muscles after modified radical mastectomy, twenty patients who had had the operation at Nagasaki Chuo National Hospital 40 days to 2 years prior to this study, were investigated by electromyography. Of 20 subjects, 11 had the operation to preserve the pectoralis major muscle and 9 had the operation to preserve both the pectoralis major and minor muscles. The results were as follows: 1) Electromyography revealed some damages to the pectoral nerves in 16 cases of 20 (80%). In 12 cases, development of fibrillation potentials was observed at rest, while motor unit potentials disappeared during voluntary contraction. These findings suggest the damage to the nerves including neutrotomy. 2) The regions with macroscopic muscular atrophy remarkably coincided with those with neurogenic changes on electromyography. This suggests that the atrophy of the pectoralis major after modified radical mastectomy might be mainly caused by the damage to the pectoral nerves. 3) The mastectomy preserving only the pectoralis major tends to cause damage to nerves innervating the sternocostal part of the pectoralis major more frequently than the mastectomy preserving both the pectoralis major and minor muscles. The abdominal part was frequently damaged irrespective of the types of the operation.