Measurements of ventilatory function, lung elastic recoil, diffusing capacity, and distribution of ventilation were obtained on healthy middle-aged cigarette smokers and nonsmokers on two occasions five years apart in order to assess the effects of smoking and the change which may occur over this five-year interval. Subjects were drawn from a randomly selected sample of the population of Tucson, AZ. Exactly the same protocol, methods, and equipment were employed in both studies. Although very few of these healthy subjects had abnormal function, there were significant differences in most indices of function between smokers and nonsmokers. However, we could discern no difference between smokers and nonsmokers in change in function over five years. It appears that, in smokers who remain free of serious respiratory trouble, there are subtle changes which accumulate over the years and which are too gradual to detect over a five-year interval.