Background: In preschoolers, performing an acceptable spirometry and measuring bronchodilator response (BDR) is challenging; impulse oscillometry (IOS) may be an alternative to spirometry. However, there is still no consensus in standardization of BDR for IOS in young children. Objective: The objective of the study was to identify optimal thresholds to define a positive BDR test with IOS. Methods: Young infants aged 3 to 6 years with suspected asthma were evaluated in a real life setting with both IOS and spirometry pre- and post-BDR. The BDR was defined as positive when the change of FEV1 was ≥12% and/or ≥200 mL. Results: Among 72 patients (age 4.98 ± 0.94 years; 64% boys), 36 (age 5.15 ± 0.99 years; 64% boys) were selected for the subsequent analysis according to ATS / ERS quality criteria of measurements. The spirometric BDR was found positive in seven subjects (19.4%). In IOS, the mean decrease in R5 and AX was 19.86% ± 10.04 and 44% ± 22.10, and the mean increase in X5 was 23.28% ± 17.82, respectively. A decrease in R5 of 25.7% (AUC 0.77, p = 0.03) and an increase in X5 of 25.7% (AUC 0.75, p = 0.04) showed the best combination of sensitivity and specificity to detect an increase of FEV1 ≥12% and/or ≥200 mL. Conclusion: The IOS may present a valid alternative to spirometry to measure BDR in preschool children. We are considering a decrease of 26% in R5 and an increase of 26% in X5 as diagnostic threshold for BDR.