Pressure-vol. (PV) curves of the lung were obtained in 2 groups of normal subjects using different esophageal balloon vols. and balloon positions. The PV curves were fitted to the exponential V = A - Be-KP. Transpulmonary pressures at different %S of total lung capacity and specific compliance were calculated. Variation in balloon vol. over the range 0.2-2.0 ml had no effect on the shape of the PV curve, as reflected by the exponential constant K, or specific compliance. Transpulmonary pressures were significantly different when the balloon vol. was 0.2 ml. With the balloon positioned at 8 or 12 cm from the gastro-esophageal junction there was no change in PV curve parameters but transpulmonary pressures and K were significantly altered at the 4 cm position. We conclude that, for clinical purposes, exact positioning of the esophageal balloon and use of the precise resting balloon vol. are not critical. Highly reproducible measurements of the elastic properties of the lung may be obtained using an esophageal balloon with a vol. of 0.6-1.0 ml and a position 8-12 cm from the gastro-esophageal junction.