BACKGROUND: Thyroidectomy is associated with a relatively high incidence of postoperative nausea and vomiting (PONV) ranging from 63% to 84%. In this study, we evaluated the safety and the antiemetic effects of tropisetron 30 microgram/kg or tropisetron 30 microgram/kg plus dexamethasone 5 mg in patients undergoing thyroidectomy under a standard anesthetic technique without narcotics. METHODS: Sixty-eight patients undergoing thyroidectomy were randomized to receive a placebo (Group C, n = 28), tropisetron 30 microgram/kg (Group T, n = 23) or tropisetron 30 microgram/kg plus dexamethasone 5 mg (Group T + D, n = 17) IV over 2 5 minutes immediately before the induction of anesthesia. The effects of these regimens on the development of PONV, adverse events and need for rescue antiemetics were analyzed for the 0 to 1 hour and 1 to 24 hours postoperative periods. RESULTS: In the 0 to 1 hour postoperative periods, the incidence of PONV in group C, T and T + D was 35.7%, 17.4% and 17.6% respectively, which showed no significant difference among the three groups (P > 0.05). In the same period, the incidence of retching or vomiting in Group C, T and T + D was 14.3%, 0% and 0% respectively, which showed a significantly lower incidence in Group T and T + D than Group C (P < 0.05). In the 1 to 24 hours postoperative period, the incidence of PONV in group C, T and T + D was 50%, 52.2% and 52.9% respectively, which showed no significant differences among the three groups (P > 0.05). During the first 24 hours postoperatively, the overall incidences of PONV was 67.9% for group C, 60.9% for group T and 58.8% for group T + D, which showed no siginificant difference among the three groups (P > 0.05). Group T + D patients had more headache compared to other groups, but there was no significant difference in theincidences of overall adverse events. CONCLUSIONS: Neither tropisetron or tropisetron plus dexamethasone was significantly different from the placebo for the prevention of PONV after thyroidectomy during the first 24 hour postoperative period. Only vomiting during the first 1 hour postoperatively was prevented in the tropisetron and combination of tropisetron plus dexamethasone groups compared to the control group.