The functional residual capacity (FRC) has been measured by gas dilution technique (GA) and body plethysmographic technique (BP) using the panting maneuver (PA). However, this maneuver is difficult to perform for patients who experience discomfort in breathing, and in cases of chronic obstructive pulmonary diseases, the FRC value measured by panting maneuver overestimates the true FRC value. Thus, in order to minimize these factors, we measured FRC during non-panting breathing (NP) using a BP device (BX-82, Minato Co., Osaka) and compared the results with the usual two methods. One hundred healthy subjects (Group I), 72 patients with restrictive ventilatory disorder (Group II, %VC = 62.7%) and 66 patients with pulmonary emphysema (Group III, FEV1.0% = 44.9%) were studied. All measurements were performed under 0.5 Hz respiratory frequency. The non-panting FRC measurement was performed by closing the mouth shutter of the BP for about 500 msec at the end of both the expiratory and inspiratory phase. From the box volume change and mouth pressure change, lisajous curves were formed and fitted by linear regression method. From this regression line, the volume of FRC was calculated. The obtained data had no significant differences among them, and there was a significant relationship between each technique. The correlation coefficient of non-panting breathing technique in Group III was slightly poor compared with the other techniques, and we therefore tried to re-classify Group III into small groups according to the severity of obstruction. It was subsequently found that this technique did not have the tendency to give a higher FRC value with increase in obstructive disorder.(ABSTRACT TRUNCATED AT 250 WORDS)