A randomized, double-blind, placebo-controlled study was carried out to investigate the efficacy of 2 mg versus 5 mg tropisetron in the prevention of post-operative nausea and vomiting. Patients between 22 and 64 years old who were undergoing surgery under general anaesthesia and who had been classified according to the American Society of Anesthesiologists criteria with physical status I or II were included in this trial. Sixty female patients were recruited and divided into three groups (n = 20 in each group). Patients in group 1 received saline solution as control, whereas in groups 2 and 3, 2 mg or 5 mg tropisetron, respectively, was administered intravenously as a single dose during the induction of anaesthesia. The number of patients vomiting within the first 2 h following the operation was significantly less in groups 2 and 3 compared with that in group 1 (one of 20 for each of the groups 2 and 3 versus 12 of 20 in group 1). Three patients required rescue anti-emetic medication in each of the groups 2 and 3. In conclusion, 2 mg tropisetron appears to be equally as efficacious as 5 mg in preventing post-operative nausea and vomiting.