The hemodynamic responses to positive end-expiratory pressure (PEEP) ventilation have been evaluated. A rise in pulmonary capillary wedge pressure which follows the use of PEEP may not reflect changes in left atrial pressure and may be associated with a fall in cardiac output. This increase in pulmonary capillary wedge pressure suggests that there is a high resistance to flow in the pulmonary circulation and that a lower level of PEEP should be used.