Impulse Oscillometry (IOS) was developed as a tool to measure lung function, and as it only requires passive cooperation, it has been successfully used in younger children. The aim of this study was to assess the utility of IOS compared to conventional spirometry for lung function measures in asthmatic Korean children aged 3 to 6 yrs. Total serum IgE levels, total eosinophil counts, and specific IgE levels were measured in 77 children with asthma and 55 control subjects. IOS and spirometry were performed in the children before and after bronchodilator administration. Asthmatic children significantly differed from control subjects in baseline resistance at 10 Hz and in their IOS-assessed bronchodilator responses through a Δresistance at 5 Hz (mean, −27.4% vs. −13.1%; p = 0.002), 10 Hz (mean, −20.4% vs. −11.5%; p = 0.012), 20 Hz (mean, −16.4% vs. −9.4%; p = 0.016), and 35 Hz (mean, −17.2% vs. −10.2%; p = 0.020). Conventional spirometry did not show statistically significant findings. Asthmatic children significantly differed from control subjects in IOS-assessed bronchodilator response through a Δresistance at 5 Hz, both in atopic children and in non-atopic children. For atopic children, atopic asthmatic children (n = 58) also significantly differed from atopic controls (n = 30) in baseline resistance at 10 Hz, 20 Hz, 35 Hz and impedance and in IOS-assessed bronchodilator responses through a Δ resistance at 10 Hz and 35 Hz. There were some significant correlations between bronchodilator responses of spirometric parameters and IOS parameters. IOS is useful diagnostic tool in early asthma development and might be a helpful objective outcome measure of early interventions.