In a double-blind investigation, comprising 300 gynaecological patients, the prophylactic use of droperidol i.v. at the start of the anaesthesia produced significant reduction in the frequency of nausea and vomiting postoperatively in the first 24 h from 34.4% to 10.3% (P=0.0001). There was no significant difference between the effect of droperidol 2.5 mg and 5 mg (P=0.45). Increased postoperative sedation was the only side-effect of any importance observed; however, this did not result in any increased period of observation in the recovery room, and the majority of patients considered it advantageous. Droperidol is recommended as a prophylactic anti-emetic for selected groups of patients, given as 2.5 mg i.v. at the start of the anaesthesia.