To obtain further information about the effects of cessation of smoking on pulmonary function, we followed subjects who attended 2 smoking cessation clinics during a period of 30 months. This paper reports the results from 15 persons who succeeded in stopping smoking for the full 30-month period and from 42 who did not succeed for more than one month. Testing included a respiratory questionnaire, spirometry, and the single-breath N2 test. Standardized methods, the same equipment, and the same experienced personnel were used throughout the study. We found that forced vital capacity, one-second forced expiratory volume, closing volume as a percentage of vital capacity, closing capacity as a percentage of total lung capacity, and the slope of the alveolar plateau of the single-breath N2 test all improved significantly in the subjects who stopped smoking. This improvement continued for as long as 6 to 8 months, and then remained stable. There was no sex difference in the response to smoking cessation, nor could we find a threshold of function below which cessation did not result in improvement. On the contrary, those subjects with the greatest impairment initially showed the greatest improvement. Respiratory symptoms virtually disappeared in those who stopped smoking. Subjects who continued to smoke showed an initial improvement in some function tests, probably due to a marked decrease in consumption, but no significant improvement during the whole period. We concluded from this study that cessation of smoking results in definite improvement in pulmonary function, that there is greater improvement in persons who begin with impaired function than in those whose function is initially normal, that respiratory symptoms disappear rapidly.