Exercise-induced bronchoconstriction (EIB) was associated with eosinophilic airway inflammation, bronchial hyperresponsiveness (BHR), atopy and airway obstruction. To understand the pathogenesis of EIB, we determine whether eosinophil is more related to the mechanism of EIB than atopy, BHR and airway obstruction. This study comprised 268 asthmatic children who underwent lung function test, methacholine challenge test, exercise challenge test, and blood tests for total IgE levels and total eosinophil counts (TEC). Urine eosinophil protein X (EPX) levels after exercise were measured by using ELISA method. EIB was observed in 195 of 268 asthmatics (72.8%). Asthmatics with EIB showed significantly increased TEC (P < 0.01) and decreased log PC(20) as compared with asthmatics without EIB. Maximal percent fall in FEV(1) after exercise was significantly correlated with TEC, log IgE, FEF(25-75%), log PC(20) (P < 0.001, respectively) and FEV(1) (P = 0.013). When the same study was carried out in nonatopic asthmatics, those with EIB showed significantly increased TEC (P = 0.01) compared with those without EIB; however, log PC(20), FEV(1), and FEF(25-75%) showed no significant differences between the two groups of nonatopic asthmatics. In addition, there was a significant correlation between the severity of EIB and TEC in nonatopic asthmatics. Urine EPX/Cr levels after exercise were correlated with the severity of EIB (r = 0.238, P = 0.014). Blood eosinophils and urine EPX/Cr after exercise correlate significantly with the maximal percent fall in FEV(1) after exercise, therefore EIB may reflect a state of eosinophilic inflammation in the airway of asthmatic children.