Preoperative lung function tests are useful to evaluate the preoperative pulmonary condition and to detect a high risk of postoperative pulmonary complications. However, maximum expiratory effort by patients is necessary to determine lung function using spirometry and flow-volume curve measurements. When patients are not able to expire completely during the measurement, incorrect data regarding their respiratory system is obtained. On the other hand, respiratory system impedance using an impulse oscillatory system (IOS) can evaluate total airway resistance (R5), large airway resistance (R20), small airway resistance (R5-20) and reactance (X5) under breathing at rest within a few minutes. There are few reports that indicate the standard values for IOS. In addition, the effects of age on IOS value are not clear. In this study preoperative lung functions using IOS were studied to examine the standard value and effect of aging.Subjects were 420 patients aged from 20 to 89 years with normal pulmonary function (%VC > or = 80%, %FEV(1.0) > or = 70%), and scheduled for an elective surgery. Lung function measurements such as IOS, spirometry, maximum expiratory flow-volume curve and single N2 washout were done preoperatively. Subjects were divided into seven groups in decades from 20 to 80.Although there was no statistical change in R5, R20, R5-R20, Z5 and X5 in the decades from 20 to 60, there were statistically significant changes during the 70s and 80s. There were significant differences in IOS parameters between the adult group and the aged group. Changes due to aging were stronger on V25/Ht than those of IOS.This study indicates that there are differences between V25/Ht and IOS values because of the difference in breathing conditions during measurements.