Forced expiratory spirograms and peak expiratory flow were measured in 102 resident male medical students (60 nonsmokers and 42 smokers). Forced vital capacity; forced expiratory volume in 1 sec; forced expiratory volume in 1 sec expressed as a percentage of forced vital capacity; forced expiratory flows between 80 and 70 per cent, between 55 and 45 per cent, between 30 and 20 per cent, and between 15 and 5 per cent of the forced vital capacity; forced expiratory time for the last 0.5 liter of the forced vital capacity; and maximal mid-expiratory flow were determined from the forced expiratory spirogram. Peak expiratory flow, all forced expiratory flows (except the forced expiratory volume in 1 sec), and the ratio of forced expiratory volume in 1 sec to forced vital capacity were significantly lower, and forced expiratory time for the last 0.5 liter of the forced vital capacity was significantly higher in the heavy smokers (those who had smoked a lifetime total of more than 10,000 cigarettes) than the nonsmokers. The light smokers (those who smoked a lifetime total of fewer than 10,000 cigarettes) had values between those of nonsmokers and the heavy smokers. Thus, a definite dose-related response to smoking was seen. Flows at lower lung volumes showed greater percentage changes than flows at higher lung volumes. The forced expiratory flow between 30 and 20 per cent of the forced vital capacity was the most sensitive test for detecting abnormality in smokers. Among heavy smokers, 58 per cent had abnormally low forced expiratory flow between 30 and 20 per cent of the forced vital capacity, whereas only 47 per cent had abnormally low ratio of forced expiratory volume in 1 sec to forced vital capacity, and 32 percent had abnormally low maximal mid-expiratory flow. The results show that even subjects with short smoking histories may have changes in pulmonary function that probably reflect narrowing of small airways. Moreover, these changes can easily be detected by simple tests, such as evaluation of a forced expiratory spirogram.