Data from a random sample of 8,191 men and women from 6 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 FEV1, which is described by a linear function of their cumulative cigarette smoking as measured in pack-years. For a typical male 173 cm tall, the estimated loss of FEV1 is 7.4 ml for each pack-year smoked. For a typical woman, 161 cm tall, the estimated effect is 4.4 ml per pack-year. Current cigarette smoking adds an acute deficit over and above the cumulative effect of lifetime smoking. For any lifetime pack-years, exsmokers have higher levels of FEV1, 123 ml for a typical man, 107 ml for a typical woman, than do current smokers of a pack per day (p less than 0.001). A man who starts smoking one pack of cigarettes per day at 25 yr of age would at age 60, after 35 pack-years of exposure, have an expected FEV1 equal to that of a man 69.4 yr of age who had never smoked. If he stopped smoking at 60 yr of age, his expected level would increase to that of a 66.5-yr-old never-smoker. This model therefore 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.