Modified radical mastectomy with preservation of one or both pectoral muscles is the surgical treatment of choice in stage I and II breast cancer. A surgical anatomic study has been performed, based on post-mortem and intraoperative findings, to investigate origin and course of lateral and medial pectoral nerves and their relations to pectoral muscles and axillary lymphonodes. The results suggest that Patey-Meier operation can produce, in 80 per cent of cases, atrophy and fibrosis of the costoabdominal portion of the pectoralis major muscle as lateral pectoral nerve is generally dissected. On the contrary, radical mastectomy with both pectoral muscles preservation permit also to preserve the nerves during lymphadenectomy if they are recognized.