Even now, the hemorrhagic syndrome after cardiac surgery with the aid or a pump oxygenator constitutes a significant problem. The purpose of this study is to postulate the possible causes of the bleeding after open hear surgery [OHS]. Fifteen consecutive OHS patients with various heart diseases were selected and platelet count, plasma fibrinogen, serum calcium level were observed pre-, intra- and post- operatively until 21 th postoperative day [POD]. The platelet count was significantly decreased with initiation of extracorporeal circulation [ECC] and continued to decrease slowly until cessation of ECC. Within 10 minutes after ECC the platelet count stared to increase. But it was significantly less than preoperative count until 5th POD. The peak count was found on 14th POD and the platelet count was gradually decreased. Plasma fibrinogen also decreased significantly during operation, but it recovered up to preoperative amount within 5 hours after termination of ECC. Thereafter it rapidly increased until 3rd POD when it reached its peak. From 3rd POD it showed slow downward slope until 21st POD, but it remained in significantly higher level than preoperative amount. Serum calcium levels showed minimum fluctuations during the whole course of study. Conclusively, the decrease in platelet count and fibrinogen amount may play a considerable role for the postoperative hemorrhage. But numerous other effects of ECC must be accounted for.