Influences of aging and cigarette smoking on the pathophysiology in elderly patients wish asthma were examined in 152 patients with asthma and 14 patients with pulmonary emphysema. The low attenuation area (LAA) <-950 HU of the lungs on high-resolution computed tomography (HRCT) was significantly larger in patients over age 70 years than in those under age 49 years. A significant increase in residual volume (RV) of the lungs and significant decrease in the value of DLco were observed in these patients over age 70 compared to the values in those under age 49. A significant correlation between % LAA and % FEV1.0 was found in patients with asthma (in both smoker and nonsmoker) and with pulmonary emphysema. However, any correlation between % LAA and the value of DLco was not observed in nonsmoking patients with asthma, suggesting that the influence of smoking on DLco is remarkably larger than the influence of aging.