Airway involvement in eosinophilic disorders other than asthma is not well-defined, and the symptoms may be overshadowed by other more prominent eosinophilic extra-respiratory manifestations. This study aimed to evaluate the utility of fractional exhaled nitric oxide (FeNO) in diagnosing eosinophilic airway involvement in patients with persistent eosinophilia (>0.5 × 10⁹/L). We conducted a retrospective analysis of adult patients with confirmed peripheral blood eosinophilia (>0.5 × 10⁹/L) on at least two occasions one month apart. Patients with blood eosinophilia associated with known eosinophilic airway inflammatory diseases were excluded from the study. Pulmonary function testing, spirometry, and FeNO measurement were conducted. A total of 14 patients with various eosinophil-related disorders were identified, with a mean age of 65.7 years. Increased FeNO levels were associated with airflow obstruction and clinical symptoms such as coughing and wheezing. Notably, eosinophil levels were not predictive of eosinophilic airway involvement. FeNO could be a useful diagnostic tool for detecting bronchial eosinophilic airway inflammation in non-asthmatic disorders, thereby enabling appropriate treatment. Further studies with larger cohorts are needed to validate these findings.