The application of forced oscillations has been introduced as a noninvasive method to measure the impedance of the respiratory system. Impedance can be partitioned into a real part or resistance and an imaginary part or reactance. The reactance depends on the elastic and inertial properties of the respiratory system. In the present study, resistance and reactance obtained in a frequency spectrum between 4 and 52 Hz were compared with maximal forced expiratory spirometry in 100 out-patients, aged 18-70 years. Resonant frequency and frequency dependence of resistance correlated significantly with selected parameters of the forced expiratory flow volume curve: correlation coefficient values ranged from 0.492 between frequency dependence and FVC and 0.668 between resonant frequency and FEV1. No correlation between average resistance and spirometric parameters reached a statistically significant level. No marked difference was observed between spirometric parameters depending on the early or effort-dependent portion and those parameters depending on the late or effort independent portion of the maximum forced expiration. It can be concluded that each measurement procedure reveals different but characteristic information about the mechanical behaviour and properties of the respiratory system.