To evaluate bronchial hyperresponsiveness (BHR) in infants with asthma and the influence of aging on BHR during the infantile period, bronchial reactivity to inhaled methacholine (BRm) in infants was monitored using the transcutaneous oxygen pressure (tcPO2) method. One hundred thirty-seven infants with asthma (from 1 to 5 years, mean 3.4 years) and 97 age-matched children without chronic respiratory diseases (from 6 months to 5 years, mean 2.1 years) were enrolled in this study. Consecutive doses of methacholine were doubled until a 10% decrease in tcPO2 from the baseline was reached. The cumulative dose of methacholine at the inflection point of tcPO2 (Dmin-PO2) was considered to represent the reactivity of tcPO2 to inhaled methacholine. Dmin-PO2 values in the asthma groups were lower than those in the control groups in each year-group from 1 to 5. There was no statistical difference in Dmin-PO2 among the 1-4-year-old asthma groups, but Dmin-PO2 in the 5-year-old asthma group was significantly lower than Dmin-PO2 in the 1-4-year-old asthma groups. The same age-related change in Dmin-PO2 was also seen in the control groups. There was no difference in age-related Dmin-PO2 change between the female group and the male group. We concluded that BRm in asthmatic children increases during the infantile period, and that the age-related changes in BRm, observed in both asthmatic and control infants, may have an effect on the clinical symptoms of asthma during childhood.