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.)