BACKGROUND: Eosinophilic inflammation of the small airways is a key process in asthma that often smolders in treated patients. The long-term effects of add-on therapy on the persistent inflammation in the small airways remain unknown. OBJECTIVE: To examine the effects of add-on therapy with either ciclesonide, an inhaled corticosteroid with extrafine particles, or montelukast on small airway inflammation. METHODS: Sixty patients with stable asthma receiving inhaled corticosteroid treatment were enrolled in a randomized, open-label, parallel comparison study of 24-week add-on treatment with ciclesonide or montelukast. Patients were randomly assigned to 3 groups: ciclesonide (n = 19), montelukast (n = 22), or no add-on as controls (n = 19). At baseline and at weeks 4, 12, and 24, extended nitric oxide analysis; pulmonary function tests, including impulse oscillometry; blood eosinophil counts; and asthma control tests (ACTs) were performed. RESULTS: A total of 18 patients in the ciclesonide group, 19 in the montelukast group, and 15 in the control group completed the study and were analyzed. With repeated-measures analysis of variance, ciclesonide produced a significant decrease in alveolar nitric oxide and a significant improvement in ACT scores over time. Montelukast produced significant decreases in alveolar nitric oxide concentrations and blood eosinophil counts over time and slightly improved ACT scores, whereas no such changes were observed in the control group. Alveolar nitric oxide concentrations with ciclesonide and reactance area at low frequencies with montelukast produced greater improvements over time compared with control. CONCLUSION: Ciclesonide add-on therapy and montelukast add-on therapy may act differently, but both separately can improve small airway abnormalities and provide better asthma control. TRIAL REGISTRATION: umin.ac.jp/ctr Identifier: UMIN000001083.