BACKGROUND: Exercise is one of the most common precipitants of acute asthma encountered in clinical practice. The development of airflow limitation that occurs several minutes after vigorous exercise, i. g. exercise-induced bronchoconstriction(EIB), has been shown to be closely correlated with the nonspecific bronchial hyperresponsiveness, which is the hallmark of bronchial asthma. All previous reports that assessed the correlation of EIB to nonspecific bronchial hyperresponsiveness have focused on airway sensitivity(PC20) to inhaled bronchoconstrictor such as methacholine or histamine. However, maximal airway narrowing(MAN), reflecting the extent to which the airways can narrow, when being exposed to high dose of inhaled stimuli, has not been studied in relation to the degree of EIB. METHODS: Fifty-six children with mild asthma(41 boys and 15 girls), aged 6 to 15 years(mean +/- SD, 9.9 +/- 2.5 years) completed this study. Subjects attended the laboratory on two consecutive days. Each subject performed the high-dose methacholine inhalation test at 4 p.m. on the first day. The dose-response curves were characterized by their position(PC20) and MAN, which was defined as maximal response plateau(MRP: when two or three data points of the highest concentrations fell within a 5% response range) or the last of the data points(when a plateau could not be measured). On the next day, exercise challenge, free running outdoors for ten minutes, was performed at 9 a.m.. FEV1 was measured at graduated intervals, 3 to 10 minutes apart, until 60 minutes after exercise. Response(the maximal DeltaFEV1 from the pre-exercise value) was classified arbitrarily into three groups; no response ((-) EIB: DeltaFEV1 <10%), equivocal response ((+/-)EIB:10% < DeltaFEV1, <20%) and definite response((+) EIB: DeltaFEV1 >20%). RESULTS: 1) When geometric mean PC20 of the three groups were compared, PC20 of (+) EIB group was significantly lower than that of (-)EIB group. 2) There was a close correlation between PC20 and the severity of EIB in the whole group(r= -0.568, p<0.01). 3) Of the total 56 subjects, MRP could be measured in 36 subjects, and the MRP of these subjects correlated fairly with the severity of EIB(r= 0.355, p<0.05) 4) The MAN of (+) EIB group was significantly higher than that of (-)EIB group(p<0.01). 5) The MAN correlated well with the severity of EIB in the whole group(r=0.546, p<0.01). CONCLUSION: The degree of MAN as well as bronchial sensitivity (PC2o) to methacholine is correlated well with the severity of EIB. The results suggest that the two main components of airway hyperresponsiveness may be equally important determinants of exercise reactivity, although the mechanism may be different from each other. The present study also provides further evidence that EIB is a manifestation of the increased airway reactivity characteristic of bronchial asthma.