Oxycodone showed similar or superior analgesic effects than fentanyl and was proposed as an alternative for fentanyl for pain control in the postoperative period. However, it was mainly directed to control visceral pain, due to the mediation of analgesic effects by the kappa receptors. However, few studies compared the oxycodone to fentanyl for postoperative pain relief in patients undergoing total hip arthroplasty (replacement). Therefore, we established this randomized study to compare the efficacy of oxycodone and fentanyl for relief of postoperative pain after total hip arthroplasty (replacement).