To evaluate the influence of aging on bronchial hyperreactivity (BHR) during the childhood period, age-related changes in bronchial reactivity to methacholine (BRm) in children from 2 to 13 yr of age were studied using the same method, employing a methacholine inhalation challenge with transcutaneous oxygen pressure (tcPO2) monitoring. Three hundred and thirty-nine asthmatic subjects (male:female = 200:139, aged [mean +/- SEM] 7.2 +/- 0.2 yr) and 107 age-matched controls (male:female = 55:52, aged 5.3 +/- 0.3 yr) were enrolled in this study. TcPO2 was measured by a tcPO2 monitor, and subsequent doses of methacholine were then doubled until a 10% decrease in tcPO2 from its baseline value was reached. The cumulative dose of methacholine at the inflection point of tcPO2 (Dmin-PO2) was considered to represent the BRm. For the sake of comparison, respiratory resistance (Rr) in subjects more than 6 yr old was measured by the oscillation technique during methacholine inhalation challenge, and the threshold point of Rr (Dmin-Rr) was also considered to represent the BRm. In the asthmatic children aged 2 to 7 yr, Dmin-PO2 decreased significantly from 12.2 +/- 2.1 to 3.1 +/- 0.8 units, but after age 8 yr the values gradually increased from 3.1 +/- 0.7 to 6.4 +/- 1.6 units. In children aged 6 to 13 yr, Dmin-Rr showed the same increase as Dmin-PO2, from 2.0 +/- 0.5 to 5.8 +/- 1.4 units. These age-related changes in BRm reflected in both Dmin-PO2 and Dmin-Rr were also seen in the age-matched controls.(ABSTRACT TRUNCATED AT 250 WORDS)