BACKGROUND: Chronic rhinosinusitis (CRS) is classified into eosinophilic CRS (ECRS) and non-ECRS. The objectives of this study were to evaluate lower airway inflammation by measuring the fractional concentration of exhaled nitric oxide (FeNO) and to examine the effects of endoscopic sinus surgery (ESS) on FeNO in patients with ECRS compared to non-ECRS. METHODS: CRS patients with nasal polyps (23 with ECRS and 22 with non-ECRS) were enrolled into this study. ECRS was diagnosed based on the definition proposed by the Japanese Epidemiological Survey of Refractory Eosinophilic Chronic Rhinosinusitis (JESREC) study group. Several clinical markers including blood eosinophil counts, percent of eosinophils in white blood cells (WBC), number of eosinophils in nasal polyps, JESREC scores, total IgE, FeNO, and Lund-Mackay paranasal sinus CT scores were compared between ECRS and non-ECRS. These markers were also tested before and 2 months after ESS. RESULTS: FeNO was significantly higher in patients with ECRS than in non-ECRS patients. When all CRS patients were tested, a significant correlation was found between FeNO and eosinophilic markers including blood eosinophil counts, percent of eosinophils in WBC, number of eosinophils in nasal polyps, and JESREC scores. FeNO showed a significant correlation with Lund-Mackay scores only in ECRS patients. Blood eosinophil counts, percent of eosinophils in WBC, and FeNO decreased after ESS only in ECRS patients. CONCLUSIONS: ECRS patients had lower airway inflammation as revealed by an elevated FeNO, which was parallel to the Lund-Mackay CT scores. ESS decreased the blood eosinophils and FeNO, leading to an improvement of the occult pulmonary dysfunction in ECRS patients.