In acute induced asthma, plethysmographic total lung capacity (TLCm) was reported to increase and lung elastic recoil [Pst(L)] to decrease. The increase in TLC is spurious (J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 52: 939-954, 1982), so that the rapid loss in Pst(L) could be due to errors in lung volume. We studied seven asthmatic patients before and during an induced bronchospasm. TLC was derived simultaneously from mouth and esophageal pressure vs. plethysmographic volume plots (TLCm and TLCes, respectively). Before bronchospasm, TLCm and TLCes were similar. During bronchospasm average TLCm increased, from 7.30 +/- 1.34 (SD) to 8.12 +/- 1.49 liters (P less than 0.001), whereas TLCes did not (P greater than 0.60). Static pressure-volume curves, derived from TLCes (P-Ves), were superimposed on prechallenge curves or only slightly shifted to the left, whereas those derived from TLCm (P-Vm) showed a clear-cut parallel shift to the left. At 70% of control TLC there was no significant change in Pst(L) measured from P-Ves curves (7.3 +/- 3.1 cmH2O before bronchospasm; 6.7 +/- 2.3 cmH2O during bronchospasm, P greater than 0.10), whereas Pst(L) measured from P-Vm curves decreased from 7.3 +/- 3.1 to 5.1 +/- 2.4 cmH2O (P less than 0.01). No significant change in Pst(L) at TLC was observed during bronchospasm. We conclude that in our patients acute decrease in Pst(L) during induced asthma was artifactual, secondary to lung volume overestimation by body plethysmography.