Platelet damage and postoperative blood loss are less severe after cardiopulmonary bypass performed with a membrane oxygenator than with a bubble oxygenator. However, this advantage of the membrane oxygenator can be partly negated by the platelet damage caused by cardiotomy suction, which implies the aspiration of air along with suction of blood. In order to reduce platelet damage by cardiotomy suction, we developed an automatic controlled cardiotomy suction system by which the aspiration of air was prevented. We evaluated platelet damage in a group of 28 patients (uncontrolled suction, n = 13; controlled suction, n = 15), and we studied the relationship between increasing volumes of cardiotomy suction and postoperative blood loss in a second group of 80 patients (uncontrolled suction, n = 47; controlled suction, n = 33). All patients underwent a coronary artery bypass operation with a membrane oxygenator. We measured significantly lower beta thromboglobulin concentrations during perfusions of approximately 2 hours and we observed a tendency toward shorter postoperative bleeding times if controlled cardiotomy suction was used. There were no significant differences between uncontrolled and controlled cardiotomy suction in platelet number and adenosine diphosphate-induced platelet aggregation. However, blood loss 18 hours postoperatively was significantly less in the controlled than in the uncontrolled suction group when the total volume of cardiotomy suction exceeded 65 L., which corresponded to perfusion times of over 3 hours. In conclusion, prevention of the aspiration of air along with suction of blood significantly reduced platelet activation and postoperative blood loss, particularly when large volumes of blood were aspirated.