BACKGROUND The aim of this study is to understand the effect of ultrasound (US) guided erector spinae plane block (ESPB) in improving the intraoperative and postoperative analgesia in patients undergoing mastectomies, decreasing the use of opioids and in reducing postoperative nausea and vomiting. METHODS After local ethics committee approval, 100 patients were divided randomly into two groups. Group A with 50 patients received US guided ESPB with 30 ml of 0.25% of bupivacaine under US guidance. Group B with 50 patients received no block. Visual analogue scale (VAS) was used to assess pain postoperatively. All patients received 1 g intravenous intravenous paracetamol 8th hourly and morphine was used as rescue analgesia if VAS score is more than 4. Patients were monitored for VAS scores, postoperative nausea/ vomiting and total morphine consumption for a 24-hour period in a high dependency unit. RESULTS Postoperative morphine consumption was found to be significantly less in patients who received US-guided ESPB compared to control group (0.12 mg ± 0.59 mg in ESPB group compared to 1.70 ± 2.29 mg which was statistically significant, p=0.000). Only 3 patients in ESP group received rescue analgesia in the form of morphine whereas 22 patients in the control group received morphine. There was no difference in PONV score in either groups. There were no complications like vascular puncture, pneumothorax, or respiratory depression in both groups. CONCLUSIONS US guided ESPB is quite effective in reducing perioperative pain in patients undergoing mastectomy. The trial was registered prospectively with CTRI with registration number: CTRI/2018/09/015668.