PURPOSE: Postoperative nausea and vomiting (PONV) occurs frequently after general anesthesia. We evaluated the timing of 2 mg iv doses of haloperidol on the efficacy of this drug as a prophylactic antiemetic for PONV. METHODS: Ninety-four non-smoking female patients with a history of motion sickness and/or PONV (Apfel's simplified risk score=3; predicted incidence of PONV=60%) were eligible to participate in this randomized, double-blind study. Patients were divided into two groups. Group 1 patients received haloperidol 2 mg iv during induction of anesthesia. Group 2 patients received haloperidol 2 mg iv approximately 30 min before the end of surgery. The occurrence of nausea and vomiting, nausea scores, pain scores, sedation levels, and adverse effects (cardiac arrhythmias, and extrapyramidal effects) were recorded by a trained, blinded investigator. RESULTS: The percentages of patients who experienced PONV in the first 24 hr were similar for the two groups (Group 1=30%; Group 2=26%, P=0.645). The incidence of PONV was significantly smaller in both groups than that predicted (60%) according to the patients' underlying risks (Group 1, P=0.004; Group 2, P=0.001). Nausea scores, pain scores, sedation scores, emergence times, and time to first rescue treatment were similar in the two groups. No adverse effects attributable to the study medication were observed. CONCLUSION: The timing of administration of haloperidol 2 mg iv does not influence its antiemetic efficacy. The recovery profiles were also similar whether haloperidol was administered at the start, or at the end of surgery.