We have performed a prospective, randomized, double-blind clinical study to assess the efficacy of ondansetron, droperidol, or both, in preventing postoperative emesis. We studied 242 patients undergoing biliary or gynaecological surgery under general anaesthesia. Shortly before induction of anaesthesia, patients received: saline i.v. (group I, n = 62); droperidol 2.5 mg i.v. (group 2, n = 60); ondansetron 4 mg i.v. (group 3, n = 57); or droperidol 2.5 mg with ondansetron 4 mg i.v. (group 4, n = 63). Nausea occurred in 45%, 37%, 32% and 29% (P = 0.234) and vomiting in 23%, 17%, 9% and 5% (P = 0.016) of patients in groups 1, 2, 3 and 4, respectively, during the first 24 h. Groups 2 and 4 had greater sedation scores than group 1 during the first 3 h (P < 0.01). We conclude that both droperidol and ondansetron showed a significant antiemetic effect, ondansetron was not significantly better than droperidol, and the combination of droperidol and ondansetron was better than droperidol but no better than ondansetron alone.