Background: Fractional exhaled nitric oxide (FE NO) and exhaled carbon monoxide (ECO) have been proposed as markers of airway inflammation and oxidative stress. Objective: The aim of this study was to assess the effect of oral prednisone treatment on FE NO and ECO levels in a group of 30 asthmatic children with asthma exacerbation. Methods: Thirty asthmatic children with asthma exacerbation were treated with oral prednisone for 5 days (1 mg/kg/day). Before and after prednisone therapy, ECO was measured by means of a chemical analyzer and FE NO was measured by means of a chemiluminescence analyzer. ECO and FE NO were also measured in a group of healthy nonatopic children. Results: Before therapy, both ECO values and FE NO values were higher in asthmatic children (ECO, 3.2 ± 0.2 ppm; FE NO online, 74.9 ± 6.2 ppb; FE NO offline, 20.2 ± 1.4 ppb) than in healthy controls (ECO, 2.0 ± 0.2 ppm [ P < .01]; FE NO online, 10.1 ± 0.8 [ P < .0001]; FE NO offline, 5.9 ± 0.4 ppb [ P < .0001]). An overlap in ECO values was found between healthy controls and asthmatic children. After prednisone therapy, there was a significant reduction in FE NO values (FE NO online, 40.6 ± 4.6 ppb [ P < .0001]; FE NO offline, 11.1 ± 0.8 ppb [ P < 0.0001]) and a slight but nonsignificant decrease in ECO values (2.7 ± 0.2 ppm [ P = not significant]) in the asthmatic group. No significant correlation between ECO values and FE NO values was found in either the asthmatic children or the controls. Conclusions: After a course of prednisone therapy, in children with asthma exacerbation there is a significant decrease in FE NO but no significant change in ECO levels. This possibly suggests that ECO is less sensitive than FE NO to inhibition by corticosteroids. (J Allergy Clin Immunol 2002;109:440-5.)