SummaryThe efficacy, safety and resource implications of a single intravenous dose of ondansetron (0.1 mg·kg −1 , maximum 4 mg) were assessed in a multinational, multicentre, randomized, double-blind, placebocontrolled trial of 427 children aged 1-12 years, undergoing tonsillectomy with/without adenoidectomy. Emesis (retching and/or vomiting) and nausea were analysed separately. Significantly more ondansetron-treated children had no episodes of emesis (127/212 (60%) vs 100/215 (47%); P=0.004) and experienced no postoperative nausea (135/211 (64%) vs 108/213 (51%); P=0.004) in the first 24 h. Ondansetron also reduced the number of emetic episodes (P<0.001), the time to the first emetic episode (P<0.001) and overall nausea severity (P=0.003). Significantly fewer ondansetron-treated children were rescued or withdrawn from the study (5% vs 10%; P=0.042). Fewer ondansetron-treated patients required nursing intervention (34% vs 45%; P=0.007) and the average intervention time was significantly shorter (4.6 vs 8.1 minutes; P=0.001). Resources used to manage PONV were significantly reduced by ondansetron (43% vs 57%; P=0.014).