In order to evaluate causes and the effective treatment of postoperative bleeding after open heart surgery, we measured platelet count and plasma fibrinogen levels before, during a 30 and 60-minute, and after a 30, 60 and 90-minute extracorporeal circulation in 5 patients perfused by a bubble and membrane oxygenator, respectively. The results were as follows: 1) The platelet count was reduced significantly by 57.2±3.04% during a 30-minute extracorporeal circulation using the bubble oxygenator and by 43.8±0.84% using the membrane oxygenator, and these levels were maintained throughout the extracorporeal circulation. 2) The platelet count after a 90-minute extracorporeal circulation increased significantly compared with after a 30-minute extracorporeal circulation (p<0.05) and these effects were more prominent using the bubble oxygenator (p<0.025). 3) The plasma fibrinogen level was reduced by 28.6±7.50% during a 30-minute extracorporeal circulation using the bubble oxygenator and by 33.6±5.77% using the membrane oxygenator, and these levels were maintained throughout the extracorporeal circulation. 4) The plasma fibrinogen level after a 90-minute extracorporeal circulation increased significantly compared with after a 30-minute extracorporeal circulation (p<0.05).