The efficacy and safety of ondansetron in preventing postoperative nausea and vomiting following minor oral surgery was evaluated in a prospective randomized double-blind study. Of a total of seventy-seven patients, randomly 38 had 4 mg of ondansetron and 39 had normal saline as placebo intravenously immediately prior to induction of anaesthesia. A standard general anaesthetic with thiopentone, suxamethonium, fentanyl, nitrous oxide and isoflurane was employed. Postoperatively nausea was assessed verbally and on a visual analog scale at 1, 4 and 24 hours from the time of awakening. Episodes of vomiting were recorded. Eight patients (21.1%) in the ondansetron group compared to 19 (48.7%) in the placebo group had nausea (P < 0.05) and 1 (2.6%) in the ondansetron group compared with 9 (23.1%) in the placebo group vomited (P < 0.05). Patients who vomited twice or more and the number who required a rescue antiemetic were significantly fewer in the ondansetron group (P < 0.05). Cardiovascular parameters were stable and showed no significant difference in the two groups. There were no significant adverse effects that could be directly attributable to ondansetron.