A prospective study of five patients was performed utilizing both left atrial and wedge catheters. As positive end expiratory pressure (PEEP) was altered, left atrial pressure, cardiac index, net left atrial pressure, pulmonary vascular resistance, and stroke work were determined. At low levels of PEEP, wedge and left atrial pressure correlated significantly (r =.83). As PEEP increased, cardiac index was frequently compromised. Measurements of net left atrial pressure and stroke work indicated that high levels of PEEP resulted in left ventricular failure. This may have been due to reduced coronary flow secondary to an elevated myocardial interstitial pressure with high levels of PEEP.