Postoperative nausea and vomiting (PONV) after adenotonsillectomy in children is, in spite of the prophylactic administration of tropisetron, still a frequent event. The aim of this study was to evaluate the benefit of the additional systemic administration of low-dose dexamethasone (0.15 mg·kg−1) for the prevention of PONV. With hospital ethics committee approval, we investigated children undergoing adenotonsillectomy receiving tropisetron (0.1 mg·kg−1; maximum dose, 2 mg) or tropisetron (0.1 mg·kg−1; maximum dose, 2 mg) plus dexamethasone (0.15 mg·kg−1; maximum dose, 6 mg) intraoperatively. The incidence of vomiting episodes and the need for postoperative analgesics were recorded. Patient data were analyzed using the t-test and the χ2 test (significance level of P = 0.05). Data values are means ± SD. Ninety children (39 girls and 51 boys), aged 5.6 ± 2.8 years and weighing 21.9 ± 8.8 kg, were enrolled in the study. The overall incidence of vomiting was 38.9% within the first 24 h (67 vomiting events) and 44.4% within 48 h postoperatively (87 vomiting events). The incidence of vomiting in the tropisetron-only group was 53.3% (24/45) at 24 h and 60% (27/45) at 48 h (24 h: P < 0.001 and 48 h: P = 0.04) and 24.4% (11/45) at 24 h and 28.9% (13/45) at 48 h in the tropisetron-dexamethasone group. The need for postoperative nalbuphine was double in patients treated with tropisetron-dexamethasone (0.61 mg ± 0.36 mg·kg−1·48 h−1) compared to that in patients receiving only tropisetron (0.31 mg ± 0.28 mg·kg−1·48 h−1; P < 0.0001). A low-dose bolus of dexamethasone (0.15 mg·kg−1) in combination with tropisetron, compared to tropisetron alone, considerably reduced the incidence of vomiting in children following pediatric adenotonsillectomy.