Bronchial hyperresponsiveness in adults is characterized by an increased sensitivity as well as an elevated maximal response to inhaled bronchoconstrictors. In children, however, it is unknown whether the maximal response increases with increasing sensitivity. We investigated the maximal degree of airway narrowing to methacholine in nonasthmatic and asthmatic children (7-12 yrs), and compared it to that in adults. Nineteen children (9 normals, 10 asthmatics) and 19 adults (8 normals, 11 asthmatics) were selected in order to cover a wide distribution of bronchial responsiveness. All subjects underwent 2 methacholine challenge tests on separate days, by inhaling doubling doses using a standardized dosimeter technique (up to a noncumulative dose of 59 mumol). The response was measured by forced expiratory volume in one second (FEV1) and expressed as a percentage fall from baseline value. The complete dose-response curves were characterized by their position (PD20, the provocative dose causing a 20% fall in FEV1) and maximal response (MFEV1, the mean response on the plateau, defined as greater than or equal to 2 points within a 5% response range). Plateaus were observed in 13 children and 9 adults, the coefficient of repeatability of MFEV1 being 10.8 and 10.4% fall, respectively. There was an inverse relationship between log PD20 and MFEV1, which did not differ between children and adults (p greater than 0.15). In most of the asthmatic children and adults the plateau could not be measured (exceeding 50% fall in FEV1) if PD20 was less than 1 mumol. We conclude that, for a given bronchial sensitivity, the maximal response to inhaled methacholine is similar between children and adults.(ABSTRACT TRUNCATED AT 250 WORDS)