Data on the relationship between exercise-induced bronchoconstriction (EIB) and exhaled nitric oxide (NO) in adult patients with asthma are controversial. It is unclear whether endogenous NO may act as either a protective or stimulatory factor in the airway response to exercise or whether changes in exhaled NO simply reflect acute narrowing of the airway. The aim of this study was to assess the changes in the fraction of exhaled nitric oxide (FE(NO)) before and after exercise challenge in patients with asthma and to analyze the relationship between FE(NO) and airway obstruction. Twenty-five non-smoking, steroid-naïve, atopic, adult patients with mild persistent asthma and 12 non-smoking, nonatopic, healthy subjects (control group) performed an exercise challenge on a cycloergometer, with monitored ventilation. FEV1 and FE(NO) were measured at baseline and 1, 5, 10, 15 and 20 minutes after the exercise challenge. Eleven of the asthmatic patients had exercise-induced bronchoconstriction (EIB group) and the remaining 14 did not (non-EIB group). Baseline FE(NO) was higher in the EIB and non-EIB asthmatic groups than in the control group. In the EIB group, FE(NO) was significantly lower 5, 10 and 15 minutes after exercise, and the changes in FE(NO) correlated with variation in FEV1 10 and 15 min after exercise. A significant correlation between baseline FE(NO) and maximal post-exercise decrease in FEV1 was found in asthmatic patients (EIB group). In conclusion, exhaled nitric oxide levels transiently decrease during exercise-induced bronchoconstriction in adult patients with asthma. Baseline FE(NO) might predict the airway obstruction resulting after exercise.