The effect of cardiopulmonary bypass on platelet count, platelet function, and bleeding time was studied in 60 patients. Platelet count was significantly reduced during and after cardiopulmonary bypass. Platelet function also had a reduced aggregation response to adenosine diphosphate. Bleeding time was prolonged to over 30 minutes during cardiopulmonary bypass and not returned to normal level until postbypass 1 hour. The amount of postoperative bleeding was proportional to the degree of decrease in platelet count and function, degree of decrease in platelet count and function. There was no significant correlation between duration of cardiopulmonary bypass and platelet count, platelet function, bleeding time, or amount of postoperative bleeding. Patients with cyanotic congenital heart disease showed a larger amount of postoperative bleeding than patients with acyanotic congenital heart disease [P<0.01], and this difference was due to the fact that platelet function was more significantly affected by cardiopulmonary bypass in cyanotic group. Patients using membrane oxygenator showed a less amount of postoperative bleeding than patients using bubble oxygenator [p<0.005] reflecting better preservation of platelet count and function by membrane oxygenator.