The influence of cigarette smoking on the pathophysiology in elderly patients with asthma. Forty asthmatics over the age of 70 years (20 ex-smokers and 20 never-smokers), and 20 patients with pulmonary emphysema over age 70 (all ever-smokers) were studied to determine the influence of cigarette smoke on the low attenuation area (LAA) <-950 HU (RA950) of the lungs on high resolution CT (HRCT) scans, and the ratio of expiratory LAA to inspiratory LAA of the lungs in relation to pulmonary function. The LAA value was significantly higher in patients with pulmonary emphysema compared with ever-smokers of asthmatics. The LAA ratio was significantly higher in ever-smokers than in never-smokers of asthmatics, and the ratio was less than 0.5 in all never-smokers, and the ratio was more than 0.5 in 10 of 20 ever-smokers of asthmatics and in all patients with pulmonary emphysema. The % RV (residual volume) was significaltly larger and % DLco (diffusing capacity for carbon monoxide) was significantly lower in subjects with the ratio more than 0.5 than in those with the ratio less than 0.5. These results suggest that cigarette smoke influences LAA of the lungs in relation to % RV value and % DLco value.