The purpose of this study was to test a plethysmographic method of measuring thoracic gas volume (TGV) that, contrary to the usual panting method, would not require any active cooperation from the subject. It is based on the assumption that the out-of-phase component of airway impedance varies linearly with frequency. By using that assumption, TGV may be computed by combining measurements of total respiratory impedance (Zrs) and of the relationship between the plethysmographic signal (Vpl) and airway flow () during forced oscillations at several frequencies. Zrs and Vpl/ were measured at 10 noninteger multiple frequencies ranging from 4 to 29 Hz in 15 subjects breathing gas in nearly BTPS conditions. Forced oscillation measurements were immediately followed by determination of TGV by the standard method. The data were analyzed on different frequency ranges, and the best agreement was seen in the 6- to 29-Hz range. Within that range, forced oscillation TGV and standard TGV differed little (3.92 ± 0.66 vs. 3.83 ± 0.73 liters, n = 77, P < 0.05) and were strongly correlated (r = 0.875); the differences were not correlated to the mean of the two estimates, and their SD was 0.35 liter. In seven subjects the differences were significantly different from zero, which may, in part, be due to imperfect gas conditioning. We conclude that the method is not highly accurate but could prove useful when, for lack of sufficient cooperation, the panting method cannot be used. The results of computer simulation, however, suggest that the method would be unreliable in the presence of severe airway inhomogeneity or peripheral airway obstruction.