Nonspecific bronchial reactivity, involving inhaled challenges with methacholine and histamine and treadmill exercise challenge, were compared in 2 groups of steroid-requiring asthmatic children (9 to 15 yr of age) distinguished on the basis of marked differences in their typical time course of clinical deterioration leading to in-hospital management. Group I patients (n = 8) had rapid clinical deterioration (occurring within 8 h from the onset of respiratory symptoms), whereas in Group II patients (n = 7), clinical deterioration occurred relatively slowly (i.e., exceeding 24 h from the onset of respiratory symptoms). Pharmacologic bronchial sensitivity was evaluated as the log dose of the agonist producing a 20% fall in FEV1 (PD20 X FEV1) and a 35% fall in specific conductance (PD35 X SGaw). Exercise-induced bronchospasm was evaluated in terms of the maximal decrease in FEV1 and SGaw after 6 min of steady-state treadmill exercise at 85% of the age-predicted maximal heart rate. Group I patients had near normal baseline FEV1 and SGaw, whereas in Group II patients, the baseline values of these parameters were significantly reduced (p less than 0.02). Group I was more sensitive to methacholine than was Group II, providing significantly lower mean values of PD20 X FEV1 (p less than 0.003) and PD35 X SGaw (p less than 0.001). On the other hand, the 2 groups were not significantly different with respect to their bronchial sensitivity to either histamine or exercise challenge. Thus, asthmatic children with marked differences in clinical presentation of bronchospasm may be further distinguished on the basis of methacholine sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)