© 2019 Taylor & Francis Group, LLC.This study aimed to postoperatively evaluate the effects of intraoperative neural monitoring (IONM) on muscles and nerves in patients who underwent modified radical mastectomy (MRM). In the 11 patients included in the study, nerves were determined and protected by nerve monitoring during the axillary dissection (IONM group). In another 11 patients, nerve monitoring was not performed; however, protection of the same nerves was attempted through careful nerve dissection (cautious nerve dissection [CND] group). The control group consisted of 22 healthy subjects. Muscle and nerve functions were blindly evaluated by an experienced physical therapy and rehabilitation specialist using electromyography (EMG) and ultrasonography (US) methods. The EMG values of the pectoralis major muscle were similar in the IONM and control groups (1.97 mV/1.98 mV, p = 0.97) but significantly lower in the CND group (1.57 mV, p < 0.05). Significant differences were found in the US values of the pectoralis major and minor muscles between the IONM and CND groups. No significant difference was found between the IONM and control groups in terms of EMG values of the serratus anterior muscle. This is the first prospective randomized study to objectively evaluate preservation of the nerve through nerve monitoring and its functional results. Monitoring of nerves during MRM is of great importance in terms of demonstrating the positive effects on muscle and nerve functions.