In a study of two groups of healthy males matched for age, smoking habits, and duration of follow-up, but differing in the rapidity of decline of FEV1 and/or vital capacity (VC), we measured lung volumes before and after bronchodilator inhalation, forced expiratory flows, diffusion capacity for CO (DLCO), and total respiratory impedance. A discriminant analysis showed that the separation between the two groups was achieved best by DLCO, weight, and FEV1/VC among the smokers and by body height and DLCO among the nonsmokers. The fact that DLCO turns out to discriminate between people with a rapid and a normal spirographic decline in the present group points to the possibility of using this measurement, in combination with FEV1/VC and weight, as a predictor of rapid decline of lung function in middle-aged healthy male smokers.