BACKGROUND: The attenuation of the lung parenchyma increases on expiration as a consequence of decreased air in the lung. Expiratory CT scans have been used to show air trapping in patients with chronic airway disease and diffuse parenchymal disease and also in asymptomatic smokers. Although there have been several reports investigating the regional air trapping on a expiratory CT scan, there have been only a few reports evaluating the changes of whole lung attenuation with considering its clinical significance, and especially in healthy subjects. The purpose of this study was to evaluate the correlation of an expiratory increase of lung attenuation with age and smoking in healthy subjects. METHODS: Asymptomatic subjects who underwent a low dose chest CT scan as part of a routine check-up and who showed normal spirometry and a normal inspiratory CT scan were recruited for this study. We excluded the subjects with significant regional air trapping seen on their expiratory CT scan. Lung attenuation was measured at 24 points of both the inspiratory and expiratory CT scans, respectively, for 100 subjects. The correlations between an expiratory increase of the lung attenuation and the amount of smoking, the patient's age and the results of spirometric test were assessed. RESULTS: There were 87 men and 13 women included in this study. Their median age was 49.0 years old (range: 25~71). Sixty current smokers, 24 ex-smokers and 16 non-smokers were included. As age increased, the expiratory increase of lung attenuation was reduced at every measuring points (r=-0.297~-0.487, Pearson correlation). The statistical significance was maintained after controlling for the effect of smoking. Smoking was associated with a reduction of the expiratory increase of lung attenuation. But the significance was reduced after controlling for the patient's age. The FEV1, FVC, FEV1/FVC and FEF25~75% were not associated with an expiratory increase of lung attenuation. CONCLUSION: The expiratory increase of lung attenuation in subjects with a normal inspiratory CT scan was negatively correlated with age. It was also reduced in heavy smokers. It may reflect aging and the smoking related changes.