Introduction: Early origins of chronic obstructive pulmonary disease have been recognized. Impulse oscillometry (IOS) is suitable for assessment of lung function also in preschool children, and some novel indices have been connected to assessment of small airway function. However, limited data exist on the sensitivity of these new indices to detect lung function deficits in young symptomatic children. Methods: IOS measurements of 103 healthy preschool children were evaluated to establish reference equations for the difference between respiratory resistance at 5 and 20 Hz (R5-20), the relative difference of R5-20 (R5-20%), and area under the reactance curve (AX). Thereafter, IOS results of children with late-onset troublesome lung symptoms (n = 20), a history of early wheeze (n = 37), or a history of bronchopulmonary dysplasia (BPD, n = 8) were compared to healthy children. Results: None of the patient groups differed from healthy regarding respiratory resistance at 5Hz (R5), and only children with a history of BPD differed from healthy regarding respiratory reactance at 5Hz (X5). In contrast, z-scores of R5-20, R5-20%, and AX were significantly higher in all patient groups than in healthy children (P < 0.001), showing improved sensitivity (20-55%) compared to R5 and X5 (5-6%). Conclusion: R5-20, R5-20%, and AX are superior to conventional IOS parameters in distinguishing children with current or past lower respiratory tract symptoms from healthy, and may prove valuable for screening early lung function deficits. (C) 2016 Wiley Periodicals, Inc.