A significant response in a bronchodilation test is one of the main diagnostic criteria of asthma. However, it is not known what the significant bronchodilator response (BDR) is by the interrupter technique (IR), measuring respiratory resistance (Rint) during tidal breathing. Fifty children with symptoms suggestive for asthma underwent an 8 min free running test outdoors. Flow-volume spirometry (FVS) and IR measurements were performed before and 10 min after running. Thereafter, all children received a salbutamol inhalation, and pulmonary function measurements were repeated 15 min later. The study population was classified into three groups according to the post-exercise pulmonary function: FEV1 > 100% of predicted (group I, n=15), FEV1 86-99% of predicted (group II, n=20) and FEV1 < 85% of predicted (group III, n=15). There were no differences in BDRs between groups I and II; the mean increases in FEV1 were 4-6% and in MMEF 20-23%, and the mean decreases in Rint were 23-26%. The mean changes in group III were significantly higher: 15% (P=0.004) in FEV1, 55% (P=0.021) in MMEF and 38% (P=0. 014) in Rint. BDR was positive (FEV1 rise >10%) in 15 children; five were in group II and 10 in group III. For MMEF and Rint, the best combination of sensitivity and specificity was achieved by a limit of 35%. By this limit, MMEF identified 14 and Rint 17 positive cases. The IR technique agreed with both FEV1 and MMEF in 73% of the cases. The IR technique provides an alternative to conventional ventilatory function measurements in bronchodilation tests in children. We suggest a decrease of 35% or more in Rint as diagnostic in post-exercise bronchodilation tests.