Rationale: Chronic obstructive pulmonary disease (COPD) includes respiratory symptoms and chronic airflow limitation (CAL). In some cases, emphysema and impaired diffusing capacity of the lung for carbon monoxide (DLCO) are present, but characteristics and symptoms vary with smoking exposure. Objective: To study the prevalence of CAL, emphysema, and impaired D L CO in relation to smoking and respiratory symptoms in a middle-aged population. Methods: We investigated 28,746 randomly invited individuals (52% women) aged 50-64 years across six Swedish sites. We performed spirometry, D L CO testing, and high-resolution computed tomography and asked for smoking habits and respiratory symptoms. CAL was defined as post-bronchodilator forced expiratory volume in 1 second divided by forced vital capacity (FEV1/FVC) < 0.7. Results: The overall prevalence was 8.8% for CAL, 5.7% for impaired D L CO (DLCO < LLN), and 8.8% for emphysema, with a higher prevalence in current smokers than in ex-smokers and never-smokers. The proportion of never-smokers among those with CAL, emphysema, and impaired D L CO was 32%, 19%, and 31%, respectively. Regardless of smoking habits, the prevalence of respiratory symptoms was higher among people with CAL and impaired D L CO than those with normal lung function. Asthma prevalence in never- smokers with CAL was 14%. In this group, asthma was associated with lower FEV1 and more respiratory symptoms. Conclusions: In this large population-based study of middle-aged people, CAL and impaired D L CO were associated with common respiratory symptoms. Self-reported asthma was not associated with CAL in never-smokers. Our findings suggest that CAL in never- smokers signifies a separate clinical phenotype that may be monitored and, possibly, treated differently from smoking-related COPD.