The low attenuation area (LAA) of the lungs on high resolution computed tomography (HRCT) was evaluated in 20 asthmatics (10 ex-smokers and 10 never-smokers) and 10 patients with pulmonary emphysema (all ex-smokers) by ventilatory function, lung volume, DLco, and a ratio of expiratory LAA/inspiratory LAA. 1 . The % LAA of the lungs on HRCT was significantly larger in patients with pulmonary emphysema(PE) than in those with asthma, but there was not significant difference in % LAA between ex-smokers and never-smokers of asthmatics. 2. A ratio of expiratory Winspiratory LAA was significantly higher in patients with PE than in those with asthma , and the ratio was significantly higher in ex-smokers of asthmatics than in never-smokers (p<0.05). 3 . The difference in % residual volume and % DLco was significant between asthma and PE , and between ex - smokers and never-smokers of asthmatics (% RV : p<0.05, DLco : p<0.05) . 4 . The difference in the values of % FVC, % FEV(1) .O, and FEV1.0% was significant between asthma and PE, but not significant between ex-smokers and never- smokers of asthmatics. The results suggest that the difference in a ratio of expiratory W inspiratory LAA, % RV and % DLco was significant between ex - smokers and never - smokers of asthmatics , and cigarette smoking induces more irreversible changes of the lungs.