BACKGROUND: Airway hyperreponsiveness is a cardinal feature of asthma. It consists of both an increased sensitivity of the airways, as indicated by a smaller concentration of a constrictor agonist needed to initiate the brochoconstrictor response and an increased reactivity, increments in response induced subsequent doses of constrictor, as manifested by slopes of the dose-response curve. The purpose of this study is to observe the relationship between bronchial sensitivity and reactivity in asthmatic subjects. METHOD: Inhalation dose-response curves using methacholine were plotted in 56 asthmatic subjects. They were divided into three groups(mild, moderate and severe) according to clinical severity of bronchial asthma. PC20 were determined from the dose-response curve as the provocative concentration of the agonist causing a 20% fall in FEV1. PC40 were presumed or determined from the dose response curve, using the PC20 and the one more dose after PC20. Reactivity was calculated from the dose-response curve regression line, connecting PC20 with PC40. RESULTS: PC20 were 1.83mg/ml in mild group, 0.96mg/ml in moderate, and 0.34mg/ml in severe. PC4O were 7.17mg/ml in mild group, 2.34mg/ml in moderate, and 0.75mg/mI in severe. Reactivity were 24.7+/-17.06 in mild group, 46.1+/-22.10 in moderate, and 59.0+/-5.82 in severe. There was significant negative correlation between PC2O and reactivity (r=-0.70, P<0.01). CONCLUSION: Accordingly, there was significant negative correlation between bronchial sensitivity and brochial reactivity in asthmatic subjects. However, in some cases, there were wide variations in terms of the reactivity among the subjects who have similar sensitivity. So both should be assessed when the bronchial response tor bronchoconstrictor agonists is measured.