OBJECTIVES: To determine the relationship between lung function impairment and quantitative computed tomography (CT) measurements of air trapping and emphysema in a population of current and former heavy smokers with and without airflow limitation. METHODS: In 248 subjects (50 normal smokers; 50 mild obstruction; 50 moderate obstruction; 50 severe obstruction; 48 very severe obstruction) CT emphysema and CT air trapping were quantified on paired inspiratory and end-expiratory CT examinations using several available quantification methods. CT measurements were related to lung function (FEV(1), FEV(1)/FVC, RV/TLC, Kco) by univariate and multivariate linear regression analysis. RESULTS: Quantitative CT measurements of emphysema and air trapping were strongly correlated to airflow limitation (univariate r-squared up to 0.72, p<0.001). In multivariate analysis, the combination of CT emphysema and CT air trapping explained 68-83\% of the variability in airflow limitation in subjects covering the total range of airflow limitation (p<0.001). CONCLUSIONS: The combination of quantitative CT air trapping and emphysema measurements is strongly associated with lung function impairment in current and former heavy smokers with a wide range of airflow limitation. KEY POINTS: - CT helps to automatically assess lung disease in heavy smokers - CT quantitatively measures emphysema and small airways disease in heavy smokers - CT air trapping and CT emphysema are associated with lung function impairment.