In order to verify the efficacy and safety of a single intravenous dose of ondansetron in the prevention of postoperative nausea and vomiting (PONV), 928 patients undergoing ophthalmic, gynaecological, abdominal and urological procedures under general anaesthesia were enrolled in a single-blind multicentre placebo-controlled study. Patients were randomly allocated to receive either a single intravenous dose of ondansetron 4mg or placebo before the induction of general anaesthesia. General anaesthesia was induced with thiopental sodium and maintained with fentanyl, nitrous oxide and isoflurane in oxygen. Postoperative analgesia was provided with the administration of morphine and/or ketoprofen, with similar total amounts being administered to both groups. The percentage of patients experiencing PONV was significantly higher in the placebo group (80vs20%; p < 0.001). 69 episodes were reported in the group treated with ondansetron compared with 227 episodes in the group receiving placebo. In a subgroup of patients with a previous history of PONV, ondansetron administration produced relief from emetic symptoms in 75% of the patients, while only 25% of patients administered placebo were symptom-free during the 24 hours of the study. No differences were observed in the incidence of adverse events between the 2 groups of patients. The results of this study demonstrate that ondansetron is effective and safe in the prevention of PONV in patients undergoing a variety of surgical procedures under general anaesthesia.