BACKGROUND: Emergence agitation is a common problem after sevoflurane anesthesia in children. Tropisetron, a 5-HT3 antagonist, significantly reduces the incidence of emergence agitation after sevoflurane anesthesia. This study evaluated the effect of ondansetron on emergence agitation after sevoflurane anesthesia in children undergoing a tonsillectomy. METHODS: Eighty children, aged 3-9 years (ASA physical status I) undergoing tonsillectomy, were randomly enrolled in this study. Group O received 0.1 mg/kg of ondansetron, and group S received 0.1 ml/kg of saline during the operation. Anesthesia was induced with 5 vol% sevoflurane and maintained with 2-2.5 vol% sevoflurane. The agitation score was recorded when they arrived at the postanesthesia care unit, and 10 minutes after that. RESULTS: The incidence of emergence agitation was 30% in group O and 27.5% in group S at arrival (P = 1.00). Ten minutes after arrival, the incidence was 12.5% in group O and 25% in group S (P = 0.25). CONCLUSIONS: Ondansetron 0.1 mg/kg does not reduce the incidence of emergence agitation after sevoflurane in children.