Studies have suggested that airway responsiveness declines with maturation; however, studies comparing infants, children, and adults are confounded by differences in size as well as maturation. Therefore, to determine whether maturation has a significant affect on airway responsiveness, we compared normal female children (n = 9; mean age = 13.6 yr) and adults (n = 7; mean age = 42.4 yr) who were matched for body size. Bronchial challenge tests were performed with increasing methacholine concentrations to a maximum of 30 mg/ml. At baseline, there were no significant differences between the two groups in lung volumes (TGV, RV, TLC) or flow-volume curves (FEV1, average forced expiratory flow rate between 25% and 75% of the vital capacity [FEF25-75], FVC). All subjects but one adolescent completed the challenge (30 mg/ml). The children had a greater percentage decline from baseline in FEV1 than the adults (17 versus 7%, p < 0.03). The percentage decline in FEF25-75 was greater for the children than for the adults, but the difference was not statistically significant (35 versus 20%, p < 0.10). Compared with the children, the adults more often demonstrated a plateau in their dose-response curves for FEV1 (22 versus 86%) and for FEF25-75 (33 versus 100%). We conclude that normal female children have a greater airway responsiveness to inhaled methacholine than do adults, and that this difference is not related to baseline lung size, airway caliber, or delivered methacholine dose.