Our study aimed to compare the efficacy of dexamethasone added to palonosetron to both palonosetron and dexamethasone monotherapy for preventing postoperative nausea and vomiting in highly susceptible patients receiving opioid-based, intravenous patient-controlled analgesia after thyroidectomy. Nonsmoking women who underwent total thyroidectomy were randomly allocated to either the dexamethasone group (Group D), the palonosetron group (Group P), or to the dexamethasone plus palonosetron group (Group DP). The severity of nausea and pain, the number of episodes of vomiting, the administrations of rescue anti-emetics, and the side effects of the antiemetics were documented in the recovery room at 2, 4, 8, 12, 24, and 48 hours after surgery. The severity of nausea was lowest in Group DP, followed by Group P and Group D. But there was an overall difference only between Group D and Group DP. The overall differences in the time to the first administration of the rescue antiemetic were observed in a Kaplan-Meier analysis (P ¼ 0.017), noting a significant difference between Group D and Group DP (P ¼ 0.003). The combination of dexamethasone and palonosetron decreased the severity of nausea and increased the time to the first antiemetic dose compared with using