BACKGROUND: Postoperative nausea and vomiting remain troublesome problems, especially in women receiving the opioid analgesics. This study was performed to assess the antiemetic efficacy of ondansetron in patients using an intravenous patient-controlled analgesia (IV-PCA) after gynecologic surgery. METHODS: In this randomized placebo-controlled study, forty healthy gynecologic surgical patients received ondansetron 4 mg or placebo at the end of surgery. Patients in the recovery room received fentanyl by PCA which provided a bolus dose of 20 microgram, a lockout time of 6 minutes, and a basal infusion of 20 microgram/hr. We assessed the occurrence of nausea, vomiting, and the need for rescue antiemetics during the first 24 hours after operation. RESULTS: During the first 24 hr after operation, 40% of patients experienced no nausea or vomiting in the ondansetron group compared to 30% of patients in the placebo group. There was no significant difference in the incidence of nausea between groups (70% in placebo group vs 60% in ondansetron group). However, ondansetron reduced the incidence of vomiting from 50% to 15%, and the need for rescue antiemetics significantly from 25% to 0% (P< 0.05). CONCLUSIONS: Ondansetron in a dose of 4 mg does not prevent postoperative nausea during the first 24 hours after operation when used with fetanyl PCA. However, ondansetron significantly reduces the chance of postoperative vomiting and rescue antiemetics.