OBJECTIVE : The aim of this study was to evaluate with electromyography (EMG) the effect of lateral pectoral nerve sparing technique (LPNST) and radiotherapy (RT) on the lateral pectoral nerve (LPN) in patients applied with modified radical mastectomy (MRM). METHODS : The study included 66 patients who underwent MRM surgery. The patients were separated into 2 groups as those applied with LPNST and those who underwent standard surgery (Control group). Within these 2 groups, patients were again separated as those who received or did not receive RT. The EMG evaluations were made by a neurology specialist blinded to the patient groups. RESULTS : The mean age of the patients was 53.3 ± 10.6 years. Standard surgery was applied to 33 (50%) patients and LPNST to 33 (50%) patients, RT was applied to 32 (48.5%) patients and not to 34 (51.5%) patients. In the EMG evaluation, latency was 2.1 ms (1.4-3.2) in the LPNST and 3.7 ms (1.9-12.4) in the control (p <0.001) and amplitude values were 9650 mV (3120-36900) in the LPNST and 4780 mV (510-12.4) in the control (p <0.001). The latency values in the Control receiving and not receiving RT were 4.0 ms (1.9-12.4) and 2.6 ms (1.9-6.2) respectively (p <0.05). The latency values of the patients receiving and not receiving RT in the LPNST were 2.2 ms (1.8-3.2) and 2.0 ms (1.4-2.4) respectively (p <0.05). In the Control and LPNST Group, no significant difference was determined between receiving and not receiving RT groups in respect of amplitude values (p >0.05). CONCLUSIONS : The results of this study demonstrated that electromyographically the latency and amplitude values were better protected in the LPNST group. It was also seen that RT increased the formation of nerve damage in both groups.