To determine bronchial reactivity patterns, 400 subjects, 5 to 21 yr of age, underwent a methacholine challenge in a Natural History of Asthma study. The diagnosis of asthma or allergy was based on a respiratory questionnaire. Subjects were nonsmokers and had had no respiratory infections for 1 month. Intradermal skin tests were done. The methacholine challenge response was expressed as the area beneath the dose-response curve (Area 35). Fifty-five asthmatics, 113 normal subjects from normal families, 103 normal subjects from asthma families, 60 normal twins, and 69 allergic subjects without asthma were studied. Overall, 52% of nonasthmatics and 47% of nonallergic subjects had an Area 35 less than 4,000 (800 breath units). There was a difference (p less than 0.05) in the distribution of methacholine Area 35 responses in normal subjects from that in normal families compared with normal subjects from asthma families. The age of the nonasthmatic subjects had an influence on the degree of bronchial reactivity. Methacholine challenge studies in pediatric patients must be interpreted with age, personal atopic status, and family asthma history in mind.