Intraluminal movements of pulmonary artery catheters are seldom considered, although they may have a significant impact on flow measurement and safety. The purpose of this study was to document the movements of these catheters within the heart cycle, and to establish whether any patterns existed. We analyzed two-dimensional real time echocardiography cross-sectional images of the human pulmonary artery in 20 patients, plotting the position of the intra-arterial catheter frame-by-frame onto a standardized template. We found that movement of the catheter within the pulmonary artery occurred in a seemingly random pattern. There was no movement pattern to the inferior right of the vessel, an area known to have complex velocity profiles and erroneous velocity measurements. The catheter moved substantially throughout the heart cycle, occasionally quite close to the vessel wall. We observed significant movement of the pulmonary artery catheter throughout the heart cycle, without any predictable pattern. Although we did not specifically investigate the effect of these movements on cardiac output, they imply that intra-arterial Doppler flow measurements from the pulmonary artery may be inaccurate, since a stable catheter position, and hence, a sample volume, cannot be ensured. Further speculation can be made on the implications of these movements in regard to thermodilution cardiac output and safety.