Objective: A variety of methods, including neuraxial and regionalblocks, have been used to manage post-mastectomy pain. Thisstudy evaluates the effect of serratus anterior plane block on acutepain after mastectomy.Methods: A total of 60 patients who were candidates for mastectomyunder general anaesthesia were divided in this trial intotwo groups. After entering the recovery room, the first-experimental-group (SAB) underwent the ultrasound-guided serratus anteriorblock, but the second-control-group (CTL) received no blockintervention. An intravenous patient-controlled analgesia (PCA)device was used to deliver fentanyl in both groups. Using the VisualAnalog Scale (VAS), the patients’ pain was assessed at intervals of 1,6, 12 and 24 hours after entering the recovery room. Intravenousacetaminophen was administered as a rescue analgesic if the painexceeded 3 on the VAS. A total amount of fentanyl consumed, thetime to first PCA request, a total amount of acetaminophen consumptionand possible adverse effects were evaluated.Results: The pain scores assessed at the time intervals were notsignificantly different between the two groups (p>0.5). The totalconsumption of fentanyl was significantly lower in the SAB groupthan in the CTL group (p=0.0001). Duration of the time to firstPCA request was remarkably longer in the SAB group than inthe CTL group (p=0.0001). The total amount of acetaminophenconsumed was notably higher in the CTL group than in the SABgroup (p=0.001). None of the patients experienced side effects.Conclusion: Undertaking serratus anterior block following mastectomycan reduce pain scores and lower opioid usage.