BACKGROUND: Although sevoflurane is almost ideal for the ihalation induction of anesthesia, considerable hemodynamic and respiratory effects have been reported during inhalation induction. The goal of this study was to evaluate the effects of low-dose fentanyl on high concentration of sevoflurane induction in adult patients with midazolam premedication. METHODS: 104 healthy patients (16 55 yr) premedicated with midazolam were randomized to receive I.V. fentanyl 0.5mug/kg (F) or placebo (P) before induction. Three vital capacity breathing with 8 vol% sevoflurane/70% N2O was applied to patients after priming of circuit with the anesthetic mixture. A blinded observer recorded the sequence of breaths (1st, 2nd, 3rd) to loss of eyelash reflex, response to verbal command (RVC), side effects, and the intubation time. MAP, HR, respiratory rate (RR), sevoflurane ET%, BIS values, SpO2, and ETCO2 were recorded at 1 minute intervals. Hypotension, tachycardia, and tachypnea were defined as a 30% decrease in MAP, a HR over 100 bpm, and a RR of over 25/min, respectively. Data were analyzed using the Chi-Square Test, ANOVA, and Post Hoc test. Results were considered significant at a P < 0.05. RESULTS: No differences in sevoflurane ET% or overall changes in MAP were observed between the two groups. HR was significantly increased in P compared to F during induction. Loss of RVC was faster in F than in P. Tachycardia and tachypnea were more developed in P than in F. BIS values were increased after intubation to a greater extent in P (from 45.8+/-18.6 to 59.2+/-20.4) than in F (from 36.0+/-9.0 to 39.9+/-10.3). Time to Intubation showed no difference between groups. CONCLUSIONS: We conclude that low-dose fentanyl can reduce the incidence of marked changes in HR and RR, and even in BIS changes at intubation during inhalation induction with sevoflurane. Fentanyl may affect the quality of sevoflurane induction rather than induction speed.