Seventeen children developed reproducible early and late asthmatic responses (dual reactions) after cycle ergometer exercise. There was a significant correlation between the magnitude of their early and late reactions, emphasizing the direct relationship of these events. No significant differences were observed in the clinical severity of asthma, diurnal variations in FEV1, and extent of the early reaction after exercise between children with dual responses and 19 children with single reactions. These findings suggest that the occurrence of late reactions after exercise is not determined by differences in severity of disease or baseline airway reactivity in the asthmatic subjects. This view is supported by the finding that there was no significant difference in the dose of acetylcholine necessary to elicit a 20% decrease in FEV1 between eight children with dual response and seven children with single early response after exercise. The rate of spontaneous recovery from early reactions was slower in children with dual responses, suggesting that this variable may predict development of late-phase reactions in exercise-induced asthma.