BACKGROUND: Aclidinium/formoterol 400/12 µg is a twice-daily maintenance bronchodilator for COPD. This post hoc study evaluated aclidinium/formoterol vs aclidinium 400 µg, formoterol 12 µg, or placebo in patient subgroups. PATIENTS AND METHODS: Data were pooled from two 24-week Phase III clinical trials (ACLIFORM and AUGMENT). Patients (N=3,394) were analyzed by baseline airflow obstruction severity (moderate/severe), age (<65/≥65 years), sex, and exacerbation history (0/≥1 exacerbation in the previous 12 months). Changes from baseline vs placebo and mono-therapies were evaluated: morning pre-dose (trough) and morning 1-hour post-dose FEV1, Transition Dyspnea Index (TDI), and moderate/severe exacerbation rates (healthcare resource utilization [HCRU] and EXAcerbations of Chronic pulmonary disease Tool [EXACT] criteria). RESULTS: Aclidinium/formoterol improved the post-dose FEV1 vs placebo and monotherapy in all subgroups (all P<0.01) and trough FEV1 vs placebo (P<0.001) and formoterol (P<0.05) across all subgroups. Improvements in trough FEV1 were observed vs aclidinium in patients with severe airflow obstruction, patients aged <65 years, males, and patients with exacerbation history (P<0.05). Improvements in TDI were observed vs placebo in all subgroups (all P<0.001), monotherapies for patients with moderate (formoterol P<0.05) or severe airflow obstruction (aclidinium P<0.05), patients aged <65 years (aclidinium P<0.01, formoterol P<0.05), males (formoterol P<0.05), and patients with no exacerbation history (formoterol P<0.05). HCRU exacerbation rates were lower for aclidinium/formoterol vs placebo in patients with no exacerbation history (P<0.01). EXACT exacerbation rates were lower for aclidinium/formoterol in patients with moderate airflow obstruction vs placebo and aclidinium, patients aged <65 years vs placebo and ≥65 years vs formoterol, males vs placebo, and patients with no exacerbation history vs placebo (all P<0.05). CONCLUSION: Aclidinium/formoterol significantly improved post-dose FEV1, trough FEV1, and TDI vs placebo across all subgroups and vs monotherapy in many subgroups. These findings further support the benefits of aclidinium/formoterol for all patients with COPD.