Patient-controlled analgesia (PCA) is a widely accepted method to control postoperative pain. Morphine is the commonest opioid for PCA, however its side effects are troublesome. Morphine-related side effects include pruritus, nausea, vomiting, constipation, urinary retention, respiratory depression, and sedation. Many studies had been conducted to reduce the side effect of morphine. The current trend of reducing morphine side effect is direct combination of other drugs and morphine in PCA. The advantages of this method are convenient in clinical use and preventive for side effects. Dexmedetomidine is a highly selec¬tive α2-adrenergic receptor agonist that has been reported to cause sedative effects and reduce opioid requirements in the perioperative period. The main advantage of this drug is that it does not cause serious respiratory side effects. The analgesic, sedative/hypnotic and anxiolytic prop¬erties of dexmedetomidine make this drug potentially useful for postoperative sedation and analgesia. No previous study had investigated the effect of direct combination of dexmedetomidine and morphine in PCA. The goal of this study is to investigate the combination effect of dexmedetomidine and morphine in postoperative patient-controlled analgesia.