The increased fraction of exhaled nitric oxide (Feno) values observed in asthmatic patients are thought to reflect increased airway inflammation. However, Feno values can be affected by airway caliber reduction, representing a bias when using Feno values to assess asthma control.We sought to determine the effect of changes in both airway caliber and inflammation on Feno values using the allergen challenge model.FEV 1 and Feno values were measured during early airway responses (EARs) and late airway responses after challenge with house dust mite allergens in 15 patients with mild allergic asthma. Helium and sulfur hexafluoride (SF 6 ) phase III expired concentration slopes (S He and S SF6 , respectively) from single-breath washout tests were measured to identify sites of airway constriction.In EARs, FEV 1 and Feno value decreases reached 36.8% and 22%, respectively (P < .001). ΔS He was greater than ΔS SF6 (+189.4% vs +82.2%, P = .001). In late airway responses FEV 1 and Feno value decreases reached 31.7% and 28.7%, respectively (P < .001), with the same ΔS He and ΔS SF6 pattern (+155.8% vs +76%, P = .001). Eight hours after the EAR, FEV 1 was still decreased (P < .001), whereas Feno values had returned to baseline. At 24 hours, FEV 1 had returned to baseline, with Feno values increased by 38.7% (P = .04).In patients with mild allergic asthma, airway caliber changes modulate changes in Feno values resulting from airway inflammation. Therefore Feno should no longer be considered solely an inflammation biomarker but rather a biomarker that integrates both airway inflammation and lung function changes. Furthermore, early and late phases resulting from allergen exposure were shown to involve similar lung regions.