The effect of cardiopulmonary bypass on platelet count and function was studied in 20 patients who underwent cardiac operation from April 1991 to August 1991 at the Department of thoracic and Cardiovascular Surgery, School of Medicine, Keimyung University. Ten patients were perfused with a bubble oxygenator, 10 with a membrane oxygenator. During and after bypass, platelet counts decreased in both groups and significantly reduced in those perfused with a bubble oxygenator. All 20 patients studied for platelet functions had an abnormal postoperative aggregation response to collagen and epinephrine, but no significant difference in both groups. One hour after bypass, bleeding times increased in both groups but did not differ significantly between groups. Postoperative 24 hour blood losses were significantly higher in patients perfused with a membrane oxygenator. Platelet damage and postoperative blood loss are less severe after cardiopulmonary bypass performed with a membrane oxygenator than with a bubble oxygenator.