The measurement of airway responsiveness in preschool children is hampered by the fact that most tests of airway caliber are difficult to carry out at a young age. Patient cooperation is only needed to a limited extent when transcutaneous oxygen tension (PtcO2) is used as an indicator of airway obstruction following bronchial provocation. In 51 children, aged 6-14 years with asthma we have measured PtcO2 and forced expiratory volume in 1 second (FEV1) concurrently after bronchial provocation, using increasing doses of methacholine administered with a De Villbiss 646 nebulizer and a French-Rosenthal dosimeter. The shapes of the dose-response curves to PtcO2 and FEV1 show a close similarity. After methacholine challenge, the decrease in PtcO2 correlates highly with the decrease in FEV1. We conclude that in children a 20% decrease in PtcO2 can be used as a sensitive indicator of airway narrowing after methacholine challenge.