The influence of smoking on pulmonary function and emphysematous changes of the lung (percentage of attenuation area < -950 HU (% LAA) on high resolution computed tomography (HRCT) was examined in 49 patients with bronchial asthma. 1. In patients with asthma, the % residual volume (RV) in many nonsmokers was less than 129%, in contrast, the % RV in many smokers was between 130% and 189% , which was higher than that in nonsmokers. 2. Significant correlations between % RV and 96LAA value, and between % RV and CT number were observed both in nonsmokers and smokers with asthma, in which as % RV more increased, % LAA value was larger, and CT number was lower. 3. % DLco value was lower in smoking patients with asthma, whose % RV was between 130% and 189% and larger than 190% , however, the % DLco value did not change in nonsmoking patients despite of higher valure in % RV. 4. A significant correlation was also observed between % FEV1.0 value and % RV both in smoking and nonsmoking patients with asthma; as % RV value more increased, % FEV1.0 value was lower. 5. Any correlation between % FVC value and % RV was not observed. These results suggest that smoking affects the % LAA of the lung on HRCT and % DLco in patients with asthma.