Measurements of ventilatory function, distribution of ventilation, diffusing capacity, and lung mechanics were made on healthy middle-aged smokers and nonsmokers drawn from a randomly selected population in order to assess the effects of cigarette smoking and the interrelationships of the several indices of lung function. Although very few subjects had abnormal function, there were significant differences in most indices of function between smokers and nonsmokers. For the total group studied, there were significant correlations between various indices of function. A significant proportion of the variance in diffusing capacity and in diffusing capacity per liter of lung volume can be accounted for by an index of lung recoil which may, in turn, be related to size of terminal air spaces.