Background: Small airway dysfunction in asthma can be measured by impulse oscillometry (IOS), where sometimes the reactance can exhibit an inversion of the curve, and its correction can determine a new value for X5: approximate X5 (X5 approx.). Our hypothesis is that X5 approx. exhibits a closer association with parameters of airway dysfunction in the IOS than X5. Methods: We analyzed 403 children (3-17 years old) who performed IOS (Sentry Suit, Vyntus®) and spirometry, recording R5, AX, X5, X5approx., Fres, D5-20 and FEF25-75. Groups X5 and X5 approx. were compared with respect to the percentage of abnormal IOS parameters, their averages, FEF25-75, and their correlation with each IOS parameter. Also, we explored the correlation between X5 and X5 approx. with each IOS parameter. Results: We found a significant decreasing prevalence of X5 approx. with age (84.6% in preschoolers, 67.2% in schoolchildren, and 36.5% in adolescents, p for trend <0.001). The preschoolers and schoolchildren with X5 approx. exhibited significant (p<0.05) alterations in many other IOS parameters (e.g. R5, Fes, AX, and D5-20) compared with those with X5. Adolescents exhibited a significant (p<0.01) alteration only for D5-20. The means of R5, AX, and D5-20 were significantly (p<0.01) higher in children with X5 approx. than with X5. Finally, in the all the age categories, compared with X5, X5 approx. correlated better with other IOS parameters and FEF25-75. Conclusion: The presence of X5 approx. decreases with age and correlates more closely than X5 with other IOS parameters for the evaluation of small airway dysfunction