BACKGROUND: Premedication with midazolam is commonly used in children to reduce anxiety and improve cooperation before anesthesia. However, it has the potential to alter respiratory function because of its muscle relaxant properties. We assessed functional residual capacity (FRC), ventilation homogeneity, using a lung clearance index (LCI), and respiratory mechanics in children awake and 20 min after oral premedication with midazolam (0.3 mg/kg).METHODS: FRC and LCI were measured using a SF6 multibreath washout technique while respiratory resistance and elastance were extracted from the input impedance obtained by forced oscillation technique in 18 children (3–8 yr) before and after oral premedication with midazolam.RESULTS: Premedication led to a small (6.5%) but statistically significant decrease in group mean FRC from 25.0 (sd 1.4) to 23.4 (1.9) mL/kg and an associated increase in LCI by 7.8% from 6.4 (0.4) to 6.9 (0.4), indicating increased ventilation inhomogeneities. Furthermore, midazolam resulted in a statistically significant increase in respiratory resistance by 7.4% from 3.38 (0.6) to 3.62 (0.6) cm H2O s/L (P <0.001) and in respiratory elastance by 9.2% from 48.8 to 52.9 cm H2O s/L (P <0.001). The changes in FRC, LCI, resistance and elastance were significantly correlated (P <0.001).CONCLUSIONS: In children with normal lungs, premedication with a relatively small-dose of midazolam led to mild changes in respiratory variables shortly after its administration. However, the anesthesiologist should be aware that using midazolam in children at high risk of respiratory complications under anesthesia might lead to a greater decrease in respiratory function.