BACKGROUND Combination therapy with a long-acting antimuscarinic agent and a long-acting β2-agonist are recommended in chronic obstructive pulmonary disease (COPD) if control is not adequate with one long-acting bronchodilator alone. We evaluated the effects of indacaterol and tiotropium combination therapy, including the effects of adding indacaterol to tiotropium (indacaterol add-on group) and adding tiotropium to indacaterol (tiotropium add-on group). METHODS We recruited 79 patients with COPD already treated with tiotropium or indacaterol. We undertook pulmonary function tests, the COPD assessment test (CAT), and the multi-frequency forced oscillation technique (to measure respiratory resistance and reactance) before and after 8 weeks of indacaterol and tiotropium combination therapy. RESULTS The median age was 72.1 years and the mean forced expiratory volume in 1 s (FEV1) as a proportion of predicted was 57.2 ± 18.3%. After 8 weeks of combination therapy, FEV1 and %predicted FEV1 had increased significantly. There was no change in CAT score. For respiratory impedance, combination therapy improved resistance at 5 Hz (R5) and resistance at 20 Hz (R20) in the whole-breath, inspiratory and expiratory phases, and resonant frequency (Fres) in the inspiratory phase. The indacaterol add-on group (43 patients) and tiotropium add-on group (36 patients) showed improvements in FEV1 and %predicted FEV1 over monotherapy, although the CAT score fell significantly in the indacaterol add-on group (p = 0.005). CONCLUSIONS Indacaterol and tiotropium combination therapy improved airflow limitation and respiratory resistances. Adding indacaterol to tiotropium, or tiotropium to indacaterol, had similar effects on airflow limitation.