The pressure-volume (P-V) diagram of the human lung was recorded on three subjects at minute ventilation from 2.5 to 180 liters/min. The area included between the inspiratory and expiratory curve is the expression of the work necessary to overcome a) airway resistance to the flow, b) lung viscosity, and c) eventual pulmonary hysteresis. From the experimental data the mean pressure (Pm) and the mean flow (Vm) have been calculated, and the mean pressure plotted against the mean flow; the extrapolation of the Pm data to Vm = o leads to a positive value of Pm of 0.5-0.9 cm H2O, and this is interpreted as being due to pulmonary hysteresis. This is almost equal to the pressure necessary to overcome the airway resistance and the lung viscosity during respiration at rest. Submitted on April 17, 1961