Abstract Background ‘Clinically important deterioration’ (CID) is a composite endpoint measuring worsening of the key clinical features of chronic obstructive pulmonary disease (COPD), namely lung function, patient-reported outcomes, and exacerbations. ACLIFORM and AUGMENT were two 24-week, randomized, double-blind, phase III studies assessing twice-daily (BID) aclidinium bromide (AB) 400 μg/formoterol fumarate (FF) 12 μg. This pooled post-hoc analysis assessed the effects of AB/FF 400/12 μg on both first and sustained CID events versus placebo and monotherapies in patients with moderate to severe COPD. Methods A first CID event was defined as the occurrence of a moderate/severe exacerbation or the worsening from baseline in ≥1 of the following: trough forced expiratory volume in 1 second (FEV1; ≥100 mL), Transition Dyspnea Index (TDI) focal score (≥1 unit), or St George’s Respiratory Questionnaire (SGRQ) total score (≥4 units). A ‘sustained’ CID was defined as a worsening maintained at all subsequent visits from appearance to week 24 or a moderate/severe exacerbation at any time. CID events were assessed at three visits (weeks 4, 12, and 24); trough FEV1 was also measured at weeks 1 and 18. Results AB/FF 400/12 μg reduced the risk of a first CID event by 45% versus placebo (hazard ratio [HR] 0.55, p