BACKGROUND AND OBJECTIVE:Combining a long-acting muscarinic antagonist with a long-acting β₂-agonist has been shown to be pharmacologically useful in patients with chronic obstructive pulmonary disease (COPD). The aim of the present study was to evaluate the effectiveness of the dual bronchodilator therapy on airway dimensions in COPD. METHODS:Patients (n = 54) were randomly assigned to receive tiotropium (18 μg once daily), indacaterol (150 μg once daily) or tiotropium plus indacaterol for 16 weeks. Quantitative computed tomography (CT), pulmonary function and health status (St. George's Respiratory Questionnaire) were measured. RESULTS:Compared with tiotropium or indacaterol alone, combination therapy resulted in a significant decrease in percentage wall area (WA%) and wall thickness, corrected for body surface area, and an increase in luminal area (Ai/BSA). Concurrent treatment was superior to monotherapy in physiological indices, including forced vital capacity, forced expiratory volume in 1 s (FEV₁) and inspiratory capacity. The changes in WA% and Ai/BSA were significantly correlated with changes in FEV₁ (r = -0.44, P < 0.01 and r = 0.37, P < 0.01). There were more significant improvements in SGRQ scores after treatment with combined therapy than with either treatment alone. CONCLUSIONS:Concurrent therapy with tiotropium and indacaterol is effective for COPD patients to promote reduction in airway wall thickness, bronchodilation, and improvements in lung function compared with a single inhaler.