Background: Objective lung function measurements are routinely used to diagnose and manage asthma, but their utility for young children has not been defined. Objective: Bronchodilator responses were measured by means of impulse oscillometry (IOS) and compared with conventional spirometry to determine the value of lung function measures in 4-year-old asthma-prone children. Methods: The study participants were in the Childhood Asthma Prevention Study (National Institute of Health/National Institute of Allergy and Infectious Diseases) and at risk for asthma. At age 4 years, concurrent asthma was determined by using a previously validated modified American Thoracic Society questionnaire. Children performed IOS and spirometry before and after albuterol administration and underwent skin prick testing to 13 common allergens to assess atopy. IOS measures were as follows: airways resistance at 5 Hz and 10 Hz, airways reactance at 5 Hz and 10 Hz, and resonant frequency. Results: Asthmatic patients versus nonasthmatic patients significantly differed in their IOS-assessed bronchodilator responses through Δ resistance at 5 Hz (medians, 27% vs 17%; P = .02) and Δ resistance at 10 Hz (24% vs 16%; P = .03). Because atopic children who have frequent wheezing are at risk for persistent asthma, the data were analyzed in regard to atopic patients with or without asthma. IOS strongly distinguished atopic asthmatic children through Δ resistance at 5 Hz (36% vs 13%, P = .007), Δ resistance at 10 Hz (25% vs 11%, P = .02), and Δ reactance at 10 Hz (47% vs 12%, P = .03). Conventional spirometry did not establish similar statistically significant findings. Conclusion: IOS bronchodilator responses are remarkably abnormal in 4-year-old children, who are most likely to have persistent asthma. IOS is a useful diagnostic tool in early asthma development and might be a helpful objective outcome measure of early interventions. (J Allergy Clin Immunol 2003;112:317-22.)