BACKGROUND: Ondansetron is a specific 5-hydroxytrypamine (HT3) receptor antagonist, sodium channel blocker and mu-opioid receptor agonist. Prophylactic intravenous administration of ondansetron has an antiemetic effect in general and epidural anesthesia. This study is designed to evaluate the antiemetic effect of intravenous ondansetron in patient-controlled epidural analgesia (PCEA) patients. METHODS: Sixty ASA physical status I-II patients undergoing elective cesarean section under epidural anesthesia using 0.75% ropivacaine and fentanyl 50microgram were received intravenous fentanyl 50microgram plus ondansetron 2 mg (group 2 mg: n = 20), 4 mg (group 4 mg: n = 20) or 8 mg (group 8 mg: n = 20) after delivery of baby. PCEA was started using 0.15% ropivacaine and 50microgram/ml butorphanol (total volume: 300 ml, 4 ml of bolus dose, and 10 min of lockout interval). The intraoperative and postoperative incidence and severity of nausea and vomiting were recorded using 4 point scale (0: none, 1: mild, 2: moderate, 3: severe) for postoperative 24 hours. RESULTS: There were no significantly lower incidence and severity of nausea and vomiting in group 8 mg (10%, 5%) than group 2 mg (25%, 10%), and group 4 mg (20%, 10%) during postoperative 24 hours. CONCLUSIONS: Prophylactic intravenous ondansetron 8 mg injection with PCEA drug has no superior antiemitic effect than 2 mg or 4 mg in cesarean section patients under PCEA without significant side effects.