The prevalence of exercise-induced bronchospasm (EIB) in asthmatic individuals has been reported to vary from 40% to 90%. There are, however, few studies addressing the effects of asthma severity on airway responsiveness to exercise. The purpose of the present study was to investigate the effects of asthma severity on EIB in children. We studied 164 children classified as having intermittent (n = 63), mild persistent (n = 30), moderate persistent (n = 40), and severe persistent asthma (n = 31) according to the Global Initiative for Asthma classification. Subjects exercised for 6 min on a cycle ergometer at 80% of their maximum heart rate, and spirometry was performed before and 5, 10, and 20 min after exercise challenge. The prevalence of EIB in children with moderate or severe persistent asthma was significantly greater than in children with intermittent asthma (p < 0.001). EIB-positive children with intermittent asthma exhibited smaller changes in FEV₁ than children in the other three groups (p < 0.001). There was no significant relationship between baseline FEV₁ and the decline in FEV₁ after exercise. We conclude that the prevalence of EIB is greater in children with more severe asthma, and that the intensity of response to exercise is not consistently related to the clinical severity of asthma.