In this randomized study we compared the efficacy of ondansetron 4 mg with ondansetron 8 mg for the prevention of postoperative nausea and vomiting (PONV) after laparoscopic cholecystectomy with sevoflurane and remifentanil infusion anaesthesia. Sixty patients were randomized to receive ondansetron 8 mg (30 pts) or ondansetron 4 mg (30 pts) before the induction of anaesthesia with thiopental and remifentanil. Anaesthesia was maintained with sevoflurane (0.5 MAC), oxygen and remifentanil infusion (0.25, 0.35, 0.5 microg/kg/min). Postoperative analgesia was provided by intravenous ketorolac 60 mg. The incidence of PONV, the pain score, and the analgesic requirement were recorded for 24 hours. There was no difference among groups in patient characteristics, risk factors for PONV, or side effects. During the first 6 h postoperatively, the incidence of PONV after ondansetron 4 mg and 8 mg were similar (p < 0.001). After 6 h the incidence of PONV increased significantly in patients who had received ondansetron 4 mg (p = 0.01) and was greater than that in patients who had received ondansetron 8 mg (p = 0.001). We conclude that single-dose ondansetron 8 mg is more effective than ondansetron 4 mg in the prevention of PONV after laparoscopic cholecystectomy. This surgery is associated with a high incidence of postoperative nausea and vomiting. A single dose of IV ondansetron 8 mg is well tolerated and decrease the number of nausea and vomiting episodes after surgery.