We utilized pulmonary artery pressure monitoring system in risky patients for preventing the postoperative pulmonary hypertensive crisis and for sampling the mixed venous blood. And this mixed venous blood oxygen saturation [MVSO2] or partial pressure [MVPO2]tells us many meaningful patients state. We selected 59 cyanotic congenital heart diseased patients, who were operated in our hospital from Nov. 1987 to Oct. 1988, in the Department of Thoracic and Cardiovascular Surgery, Seoul National University Childrenulcorner Hospital, who had pulmonary artery pressure monitoring catheter and who made us know their mixed venous oxygen condition. We found that there was no close relationship between MVPO2 and Cardiac Index [C.I.] during early postoperative period, but on the first and second day after operation the correlation coefficient was increased as r=0.35[p=0.008], r=0.78[p=0.0001]. So we concluded that the correlation between MVPO2 and C.I. was more reliable with time going as hemodynamic stabilization. And we experienced no survivors whose MVPO2 was under 20 torr, but that was not the only factor for death. From these results, we conclude that we can consider the MVPO2 [or MVSO2] representing C.I. after stabilized postoperative condition of the open heart surgery patients, but during early postoperative period, in addition to this MVPO2, we should do also apply other parameter such as urine output, arterial blood pressure, left atrial pressure and pulmonary arterial pressure for exact estimation of the patients status.