To determine whether the decline in pulmonary function in smokers is modified by stop-smoking intervention, a randomized controlled study (the Multiple Risk Factor Intervention Trial) was done comparing participants in a special intervention group that included an intensive smoking cessation program with those assigned to usual care. The subjects were 6,347 middle-aged male smokers who had serial measurements of pulmonary function--principally the forced expiratory volume in 1 second (FEV1)--during 6 to 7 years of follow-up. No overall differences were detected in the rate of loss of FEV1 in the two groups. The use of beta-blockers, which had detrimental effects on FEV1, was significantly more common in the intervention group. Among nonusers of beta-blockers, heavy smokers lost FEV1 at a rate about 11 ml per year slower in the intervention group than in the control group (2P = .09) and ended the trial with an FEV1 about 90 ml higher (2P = .05). These results support the inference from observational studies that smoking cessation has a beneficial effect on pulmonary function in heavy smokers.