The single-breath carbon monoxide diffusing capacity (DL) was measured together with standard spirometry as part of a survey of a randomly selected community population sample. Based on answers to a self-administered questionnaire, subjects free of respiratory symptoms or disease were identified. Data from subjects who had never regularly smoked cigarettes had been used to derive reference equations for the test variables, and data from the remaining subjects who had smoked were examined to determine the effect of smoking and smoking cessation on the DL. From this cross-sectional analysis, it is apparent that cigarette smoking is associated with a decrease in DL that occurs very soon after beginning the cigarette habit. There is an irreversible decrease in DL with cumulative cigarette consumption, but also a reversible phenomenon that leads to rapid improvement in DL on smoking cessation.