BACKGROUND: This study was designed to determine the effectiveness of the continuous infusion of ondansetron for the prevention of postoperative nausea and vomiting (PONV) in intravenous patient-controlled analgesia (PCA). METHODS: One hundred and sixty patients undergoing spinal surgery were randomized into four groups according to the method of ondansetron administration, placebo (n = 40, group 1), ondansetron 8 mg mixed to IV PCA (n = 40, group 2), ondansetron 4 mg IV before induction or after surgery in addition to 8 mg mixed to IV PCA (n = 40, group 3 or n = 40, group 4). The incidences of nausea, vomiting, and side effects were recorded for 48 hr postoperatively. RESULTS: The incidence of nausea in group 1 (43 %) was significantly higher than in the other groups (group 2; 18%, group 3; 15%, group 4; 18%) (P < 0.05), and vomiting was one in group 1. CONCLUSIONS: Continuous ondansetron infusion is effective at preventing PONV, but the effects of additional bolus injections to continuous infusion of ondansetron were not different from continuous infusion only.