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The authors assess the utility of postexpiratory high-resolution computed tomography (HRCT) scans in the evaluation of air trapping in patients with obstructive lung disease. Thirteen healthy volunteers and 14 patients with obstructive lung disease (OLD) were examined using inspiratory and postexpiratory HRCT scans. All had pulmonary function tests. Lung attenuation was measured on both inspiration and expiration, and the extent of air trapping on postexpiratory scans was visually assessed. The results of the healthy volunteers were compared with those of patients with OLD, and HRCT findings were correlated with pulmonary function tests. Lung attenuation on expiration, its change from inspiration to expiration, and air-trapping scores of patients with OLD were significantly different from those of the healthy volunteers. When the healthy participants and patients with OLD were combined and analyzed as one group, the lung attenuation change and air-trapping score both significantly correlated with pulmonary function tests (p < 0.001) and correlation values were similar (r = 0.61–0.89). Air trapping was found in 61% of healthy participants and all the patients with OLD, although the extent was significantly greater in the patients with OLD. Inspiratory and postexpiratory HRCT can be used to evaluate air trapping in patients with OLD. Measurements of lung attenuation and estimates of air-trapping extent are both valuable methods of analysis.