An energy analysis and data from the literature on the relation among surface area, recoil pressure, and lung volume are used to calculate the surface tension-surface area curves corresponding to pressure-volume loops. The energy analysis has been described earlier (J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 50: 921-926, 1981). It is based on the assumption that the tissue structure of the lung constitutes a conservative mechanical system and hence that pressure-volume hysteresis is primarily a result of surface tension-surface area hysteresis. Unlike previous methods of calculating surface tension from recoil pressure, this method does not rely on the assumption that the tissue component of recoil in the air-filled lung is the same as recoil pressure of the saline-filled lung at the same lung volume. The calculated values of surface tension decrease to less than 2 dyn/cm as surface area decreases along the deflation limb of the pressure-volume curve. Surface tension increases very steeply with surface area on the inflation limbs, reaching a limiting value of just under 30 dyn/cm. The shape of the surface tension-surface area curves, unlike the shape of the curves calculated by previous methods, is similar to the shape obtained on surface tension balances for fluid extracted from lungs.