The aim of the present study was to determine reference values and predictive variables for respiratory impedance (Z(rs)) by the forced oscillation technique (FOT) in subjects aged >65 yrs. The investigation involved a prospective study of nonsmoking subjects, with normal forced expiratory volumes. The Z(rs) parameters, which included average resistance between 4-16 Hz (R(4-16)), average resistance between 4-30 Hz (R(M)), resonant frequency (FN), capacitance (C) and inertance (I), were measured along with forced expiratory manoeuvres. Every subject had each parameter measured in the same sequence using FOT and spirometry. A total of 223 subjects aged 83+/-8 yrs were included in the study. The mean values for forced expiratory volume in one second (FEV(1)) % predicted were 110+/-23. The forced vital capacity (FVC) % pred was 114+/-21 and the FEV(1)/FVC % pred was 112+/-11. The mean values for the Z(rs) parameters were: R(4-16): 0.25+/-0.07 kPa.s(-1).L(-1); R(M): 0.25+/-0.06 kPa.s(-1).L(-1); FN: 11.0+/-2.8 Hz; I: 1.17+/-0.26 Pa.L(-1).s(-2); and C: 20.5+/-9.0 mL.hPa(-1). In multiple regression models adjusted for age, sex, height and weight, height was the most influential predictor for Z(rs) parameters based on the magnitude of the regression coefficient. In conclusion, it was found that height was the best predictor for respiratory impedance parameters. Contribution of age and weight was negligible. However, the level of predictability for respiratory impedance parameters by regression equations was low.