It is known that exhaled nitric oxide (ENO) is increased in asthmatic individuals, probably as an expression of airway inflammation, but no studies have been reported of ENO and exercise-induced bronchoconstriction (EIB). We assessed the effect of a treadmill exercise challenge on ENO concentration in 24 asthmatic children aged 11.2 +/- 0.4 yr (mean +/- SEM). According to the presence or absence of EIB, the children were divided into an EIB group (n = 10) and a non-EIB group (n = 14). ENO was measured with a single-breath reservoir technique. FEV(1), ENO, and heart rate were measured at baseline and 1, 6, 12, and 18 min after the end of exercise. We also measured ENO in 18 healthy control children aged 10.8 +/- 0.6 yr, of whom nine underwent an exercise challenge identical to that of the asthmatic children. After the exercise test, the mean decrease in FEV(1) was 34% in the EIB group and 5% in the non-EIB group. The EIB group had higher baseline ENO values (12.3 +/- 1.6 ppb) than the healthy children (6.1 +/- 0.2 ppb) (p < 0.01). The time course of ENO was similar in the EIB, non-EIB, and control groups, with no significant changes after exercise (p = NS). In the overall group of asthmatic children there was a significant correlation (r = 0.61, p < 0.01) between baseline (preexercise) ENO and magnitude of the maximal decrease in FEV(1) after exercise. In conclusion, our study shows that ENO levels do not change during acute airway obstruction induced by exercise challenge in asthmatic children. In addition, baseline ENO values correlate with the magnitude of postexercise bronchoconstriction, suggesting that NO may be a predictor of airway hyperresponsiveness to exercise.