Data from a random sample of 8191 men and women from six U.S. cities are used to fit a model describing the effects of cumulative and current cigarette smoking on pulmonary function. The data show that smokers suffer an irreversible loss of FVC and FEV(1) which is described by a linear function of their cumulative cigarette smoking, for example, as measured in pack-years. For a typical male 173 cm tall, the estimated loss of FEV(1) is 7.5 ml for each pack-year smoked. For a typical woman, 161 cm tall, the estimated effect is 4.5 ml per pack/per year. Current cigarette smoking adds an acute deficit over and above the cumulative effect of lifetime smoking. For any lifetime pack-years, ex-smokers have higher levels of FEV(1), 127 ml for a typical man, 106 ml for a typical woman, than current smokers of a pack per day (p < .001). A man who starts smoking one pack of cigarettes per day at age 25 yr would at age 60 yr, after 35 pack-years of exposure, have expected FEV(1) equal to that of a man aged 69 yr who had never smoked. The model estimates how much lung function is irreversibly lost by smoking, estimates how much could be regained with cessation of smoking, and predicts the future loss of lung function in both cases.