Mechanical impedance of the respiratory system was measured at 5, 10, 15 and 20 Hz in 47 healthy non-smokers, 20 ex-smokers and 51 current smokers. Besides the resistance and its frequency dependence, the compliance, the inertance and the resonant frequency of the respiratory system were computed using the assumption of a second order linear system. The data were normalized for sex, age and body height on the basis of correlations observed in non-smokers. No significant difference was found in ex-smokers. In smokers the resistance was significantly increased at all frequencies. However, it was normal in a number of subjects with abnormally low maximal expiratory flows. unlike maximal flows, forced oscillations indices were not correlated to tobacco consumption expressed in pack-years. The results do not indicate that impedance measurements breathing air may be specially useful for early detection of airway abnormalities.