Exhaled carbon monoxide (CO) concentrations were measured on a CO monitor by vital capacity maneuvers in asthmatic patients receiving or not receiving inhaled corticosteroids and in nonsmoking and smoking healthy control subjects. CO was detectable and measured reproducibly in the exhaled air of all subjects. The exhaled CO concentrations were higher in asthmatic patients not receiving inhaled corticosteroids (5.6+/-0.6 ppm, p < 0.001) and similar in asthmatic patients receiving inhaled corticosteroids (1.7+/-0.1 ppm) compared with those in nonsmoking healthy control subjects (1.5+/-0.1 ppm). Smoking healthy control subjects had the highest levels of exhaled CO concentration among the groups (21.6+/-2.8 ppm, p < 0.001). To examine whether inhaling corticosteroids reduce exhaled CO concentration in a given asthmatic patient, 12 patients with symptomatic asthma who were being treated by inhaled beta2-agonists alone underwent measurements of exhaled CO concentration before and 4 wk after the initiation of inhaled corticosteroid treatment. All patients had reductions in exhaled CO concentration (p < 0.001) and eosinophil cell counts in sputum (p < 0.01) that were accompanied by an improvement in airway obstruction. Changes in exhaled CO concentration were significantly related to those in the eosinophil cell counts in sputum (p < 0.001). The present study shows an elevation of exhaled CO in asthmatic patients that decreases with corticosteroid therapy. Increases in the exhaled CO levels therefore may reflect inflammation in the asthmatic lung.