To assess their value for early recognition of airway abnormalities, forced expiration slope ratios as recently defined by Mead, were measured at 5 lung volumes in 114 healthy nonsmokers and in 76 cigarette smokers. In healthy nonsmokers, slope ratios were independent of sex and body height, but were significantly correlated with age. They tended to be higher at low lung volume, particularly in older subjects. In male smokers with a tobacco consumption of less than 5 pack-years, slope ratios were increased at both ends of the volume range, suggesting nonhomogeneous lung emptying and peripheral airway obstruction. With increasing tobacco consumption, slope ratios tended to return to normal values at low lung volume, perhaps because of airway closure. In female smokers, slope ratios were decreased at high lung volume and increased at low lung volume, suggesting that both central and peripheral airways were abnormal. Mead's analysis appeared to be a useful tool for discussing the data in terms of functional abnormalities. However, because of their large variability, slope ratios were not as successful as transit time indices for recognizing abnormal subjects.