Esophageal elastance (Ees) was measured in 13 human subjects, awake and anesthetized by halothane, in the supine and left lateral postures. Static esophageal pressure was measured by an esophageal balloon catheter, respiratory volumes by pneumotachography, and functional residual capacity (FRC) by body plethysmography. In the supine awake subject, Ees averaged 3 cmH2O/ml at FRC and increased significantly with lung volume. Ees remained unaltered at FRC during anesthesia in the supine position, but lung volume dependence was no longer observed. In the left lateral position the awake subject displayed the same Ees at FRC as in the supine position and lung volume exerted only a small influence on Ees. The latter was reduced during anesthesia and tended to diminish with increasing lung volume. All variations in Ees caused by posture, anesthesia, and lung volume were small and affected the shape of the pressure-volume curve of the lung to only a minor degree.