Results of certain tests of pulmonary function, including a questionnaire, single-breath N2 test of closing capacity, forced expiration, and diffusing capacity were significantly different in groups of male smokers and nonsmokers. The influence of age on these smoking-related changes of pulmonary function was evaluated. The analyses indicated that (1) some tests including number of symptoms; closing capacity, i.e., closing volume plus residual volume as a percentage of total lung capacity; residual volume as a percentage of total lung capacity; Phase III of the single-breath N2 test, and steady-state diffusing capacity (ml of CO/mm Hg - min) revealed significant differences between adjusted mean smoker and nonsmoker values but did not reveal differences associated with age. (2) Tests of forced expiration (1-sec forced expiratory volume/vital capapity, reciprocal of the maximal mid-expiratory flow, maximal flow at 50 per cent of vital capacity; and moments) however, revealed differences between smoker and nonsmoker means )adjusted and unadjusted), as well as increasing smoker-nonsmoker differences with increasing age. It is suggested that the first group of tests probably measured an all-or-none response that occurred with the onset of smoking and was not affected by duration of smoking. The second group of tests probably measured the effects of continued smoking and indicated increasing abnormality associated with longer exposure (years of smoking). Test showing age-related differences between smokers and nonsmokers may reflect cummulative, irreversible changes in pulmonary function to a greater extent than test that do not.