Background: The aim of this study is to elucidate the treatment outcome of eosinophilic airway inflammation in patients with chronic airway diseases including non-asthmatic eosinophilic bronchitis (NAEB). Methods: Patients with sputum eosinophilia confirmed between 2012 and 2015 in Seoul National University Hospital were enrolled. We evaluated the prognosis and predictors of acute exacerbation in NAEB patients and investigated predictors of the improvement of eosinophilic inflammation in chronic airway diseases such as NAEB, asthma, COPD, and asthma-COPD overlap syndrome (ACOS). Results: In total, 398 patients with sputum eosinophilia were enrolled. Of these patients, 152 (38.2%) with NAEB were identified. Among them, 10 (6.6%) experienced exacerbations requiring treatment of antibiotics or systemic steroids. Inhaled corticosteroids (ICSs) therapy did not affect the risk of exacerbation even in the analysis with propensity score (P = 0.942). Of 176 patients with chronic airway diseases 37 (21.0%) showed improvement of sputum eosinophilia at 1 year follow-up. Patients who had persistent airflow limitation and received ICS therapy for less than 75% of follow-up days were less likely to be improved in eosinophilic inflammation (aOR, 0.26; 95% CI, 0.09-0.77). Conclusions: About 6% of NAEB patients experienced exacerbations. Majority of patients with chronic airway diseases, especially COPD/ACOS patients, if they were not sufficiently treated with ICS, were less likely to achieve the improvement of eosinophilic inflammation.