Many patients undergoing gynecologic laparoscopic surgery experience postoperative nausea and vomiting (PONV) despite prophylaxis and treatment with HT3 receptor antagonists such as ondansetron. Involvement of multiple types of receptors and factors may be a reason for inadequate control of PONV with a single agent. Betahistine, a histamine antagonist at H1 receptor and antagonist at H3 receptor, is widely used as a treatment of dizziness. Dizziness is one of the cause of nausea and vomiting. This study is to compare the effects of ondansetron and combined ondansetron and betahistine in preventing PONV in high-risk patients receiving intravenous opioid-based patient-controlled analgesia (IV-PCA) after gynecological laparoscopic surgery.