In normal subjects breathing spontaneously, complicance has been shown to be influenced by the lung volume from which deflation started. We wondered whether this would also be true for patients with acute respiratory failure who required mechanical ventilation and we accordingly studied 15 such patients. Chest inflation was performed using a continuous flow device (ATM-PV 102), and total compliance was measured as the slope of the pressure-volume relationship during deflation. As inflated lung volumes were increased by 10 to 15, 20 and 25 ml/kg body weight above FRC, mean compliance increased to 38.2, 45.5, 52.2 and 59.3 ml/cmH2O respectively. Each of these increases in mean compliance was statistically significant (p<0.001). Increasing the inflated lung volume to 30 ml/kg produced no further significant increase in mean compliance. This study showed that, in patients with acute respiratory failure requiring mechanical ventilation, compliance measured during deflation is a function of the inflated lung volume. We recommed that the compliance should be measured from a constant and high lung volume, equal to 25 ml/kg body weight above FRC.