We have previously reported a new technique of evaluating bronchial responsiveness by monitoring the transcutaneous oxygen pressure (tcPO2). This method is so simple, painless, and effortless with high reproducibility that it is possible to use the technique on children as young as 2 years old. Consequently, we used this method to study bronchial hyperresponsiveness in 141 children with asthma and 46 disease controls without asthma or chronic respiratory disorder. The bronchial responsiveness in asthmatic children aged 2 to 5 years was higher than in the disease controls (P less than .001). Further, bronchial responsiveness was significantly higher in moderately as opposed to minimally affected asthmatics (P less than .01), and was gradually higher according to clinical severity. In other age groups as well, the bronchial responsiveness of asthmatic children was higher than of disease controls. There was a close relationship between the level of increased bronchial responsiveness and the clinical severity of asthmatic children.