Two potential errors of pulmonary artery wedge pressure measurement that have received little attention are improper anatomic referencing of the transducer and nondependent placement of the pulmonary artery catheter tip. Transducers are often referenced to the midaxillary line of the supine patient. We utilized CT of the chest to determine accurately the topographic location of the left atrium to evaluate the accuracy of referencing the transducer in this position and to investigate atrial location in the lateral decubitus positions vis-à-vis external landmarks, so that more accurate referencing could be performed with a pulmonary artery catheter tip in the dependent lung. A prospective group consisting of ten patients referred for chest CT as well as retrospective review of 40 chest CT scans served as the study population. This study demonstrated a discrepancy between the perceived midaxillary line and the true location of the midleft atrium. Lateral decubitus CT scanning revealed a more readily localizable surface anatomic landmark which consistently and accurately predicts midleft atrial location. In addition, the appropriate decubitus position guarantees dependent catheter tip placement (Zone 3).