Several studies have evaluated the effect of pectoral nerve blocks to improve postoperative analgesia following breast cancer surgery resulting in contradictory findings. The aim of this study was to examine the effect of Pecs blocks on postoperative analgesia in women following mastectomies.We performed a quantitative systematic review in compliance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement. Articles of randomized controlled trials that compared Pecs block (types I and II) to a control group in patients undergoing mastectomy were included. The primary outcome was total opioid consumption 24 hours after surgery. Secondary outcomes included pain scores and side effects. Meta-analysis was performed using the random effect model.7 randomized controlled trials with 458 patients were included in the analysis. The effect of pectoral nerve blocks on postoperative opioid consumption compared with control revealed a significant effect, weighted mean difference (WMD) (95% CI) of --4.99 (-7.90 to -2.08) mg intravenous morphine equivalents (p=0.001). In addition, postoperative pain compared with control was reduced at 6 hours after surgery: WMD (95% CI) of -0.72 (-1.37 to -0.07), p=0.03, and at 24 hours after surgery: WMD (95% CI) of -0.91 (-1.81 to -0.02), p=0.04.This quantitative analysis of randomized controlled trials demonstrates that the Pecs block is effective for reducing postoperative opioid consumption and pain in patients undergoing mastectomy. The Pecs block should be considered as an effective strategy to improve analgesic outcomes in patients undergoing mastectomies for breast cancer treatment.