The mean airway pressure difference within a respiratory cycle at end-inspiration was measured at different tidal volumes (VT) in 59 patients treated with mechanical ventilation, and plotted in a pressure-volume (P-V) diagram. Regression analysis revealed three types of regression lines which were clearly correlated with clinical pulmonary condition and outcome. The slope of such regression lines may be a more realistic representation of lung elasticity than conventional total static compliance, which is measured at only one VT, and the intercept of these lines may be a more appropriate reflection of operating lung volume than functional residual capacity. Ventilator settings should put the patient's lungs on the steepest part of the P-V curve with the smallest intercept, while maintaining acceptable arterial blood gas tensions.