To quantitate the alterations in left ventricular (LV) dimensions and performance at successive levels of positive end-expiratory pressure (PEEP), 16 patients undergoing coronary artery bypass grafting (CABG) underwent instrumentation with ultrasonic dimension transducers to measure the minor-axis diameter of the left ventricle. Matched micromanometers were placed to measure intracavitary LV pressure and intrathoracic pressure. LV pressure and dimension data were recorded and computer analyzed during continuous positive-pressure ventilation at 0, 5, 10, and 15 cm H2O of PEEP 4 to 8 hours postoperatively. Preload was determined by the end-diastolic minor-axis diameter, cardiac output was measured by thermodilution, and indices of LV contractility assessed included the maximal velocity of minor-axis shortening and the slope of the end-systolic pressure-diameter relationship. PEEP produced a progressive increase in intrathoracic pressure associated with a fall in cardiac output; this was associated with a decrease in LV end-diastolic diameter and no significant change in the maximal velocity of minor-axis shortening or the slope of the end-systolic pressure-diameter relationship. Our results indicate that PEEP of 10 cm H2O or greater will produce a significant fall in cardiac output in patients following CABG, due to a decrease in preload rather than impaired LV contractility.