Proper positioning of pulmonary artery catheter and predicting distal migration of it during cardiopulmonary bypass is important for the prevention of perioperative complication of pulmonary artery rupture. The authors therefore examined the insertion length of pulmonary artery catheter via right internal jugular vein at which catheter tip was advanced to the most proximal position where pulmonary capillary wedge pressure could be obtained. And also the distal migration is measured by comparing the difference between prebypass and postbypass corrected length. Just before bypass, pulmonary artery catheter was arbitrarily receded 5cm for the prevention of spontaneous distal migration and wedging. The subjects were 47 patients who underwent open heart surgery (valve replacement, coronary artery bypass graft) during the period of November, 1992 through April, 1993. The results were as follows. 1) The insertion length of pulmonary artery catheter from skin to the right ventricle inlet, pulmonary artery inlet and pulmonary capillary wedge position were 26.5±3.0 cm, 36.8±4.9 cm and 46.1±5.7 cm respectively. 2) There were no significant statistical correlations between insertion lengths and patient constitutions such as body weight and height(r=0.144, r=0.032). 3) Locations of catheter tips were distributed to the 0.9±3.1 cm left to the spinous process, 1.9±1.6 cm inferior to the carina ; mostly LLQ of the lung field(51%). 4) During cardiopulmonary bypass, pulmonary artery catheter tips were migrated distally in 93.6% of the cases and the migration lengths were 2.7±2.0 cm. 5) There were no major perioperative complications caused by pulmonary artery catheterization. These results suggest that 5 cm withdrawal of pulmonary axtery catheter just before cardiopulmonary bypass can prevent the fatal complication of pulmonary artery rupture owing to its spontaneous distal migration.