Lack of consensus on diagnosis of ACO limits our understanding of the impact, management and outcomes of ACO. The present observational study aims to describe the prevalence, clinical characteristics and course of individuals with ACO based on various definitions used in clinical practice.We included individuals with COPD from the prospective, multisite CanCOLD study and defined subjects with ACO using seven definitions commonly used in the literature.Data including questionnaires, lung function and CT scans were analysed from 522 individuals with COPD who were randomly recruited from the population. Among them, 264 fulfilled at least one of the seven definitions of ACO. Prevalence of ACO varied from 3.8% to 31%. Regardless of the definition, individuals with ACO had worse outcomes (lung function and higher percentage of fast decliners, symptoms and exacerbations, health-related quality of life and comorbidities) than the remaining patients with COPD. Conversely, patients with non-ACO had higher emphysema and bronchiolitis scores. The three definitions that included atopy and/or physician diagnosis of asthma identified subjects who differed significantly from patients with COPD. The two ACO definitions with post-bronchodilator reversibility were concordant with COPD and were the least stable, with less than 50% of the patients from each group maintaining reversibility over visits.Atopy and physician-diagnosed asthma are more distinguishing characteristics to identify ACO. This finding needs to be validated using measures of airway inflammation and other specific biomarkers.