BACKGROUND: Remifentanil can be an alternative to N2O in general anesthesia. Intraoperative remifentanil may lead to acute opioid tolerance. This study aims to assess the effect of remifentanil substituted for 70% N2O on postoperative pain in children undergoing tonsillectomy/adenoidectomy. In addition, we evaluated the effect of remifentanil infusion on incidence of emergence agitation in these patients. METHODS: Eighty children, aged 2-12 years, undergoing tonsillectomy/adenoidectomy were randomly allocated to the N2O group (Group N; n = 40, sevoflurane and 70% N2O) or remifentanil group (Group R; n = 40, sevoflurane with remifentanil infusion at the rate of 0.17 microg/kg/min). In the recovery room, severity of pain and agitation were assessed by an investigator blinded to the assigned group. Time to eye opening and incidence of severe pain and agitation were compared between groups. Logistic regression was used to identify factors related to occurrence of severe pain and agitation. RESULTS: Number of patients with severe postoperative pain was 6 and 16 in groups N and R, respectively (P = 0.012). Incidence of emergence agitation was not significantly different between groups. Remifentanil infusion was a significant factor related to the occurrence of severe postoperative pain (P = 0.015), and age was inversely related to occurrence of emergence agitation (P = 0.001). CONCLUSIONS: In children undergoing tonsillectomy/adenoidectomy, intraoperative remifentanil infusion may increase incidence of severe postoperative pain compared to N2O, but it may not affect incidence of emergence agitation.