BACKGROUND Extracorporeal circulation with circuits coated with surface-bound heparin has allowed reduced levels of systemic heparinization. Clinical benefits have included reduced postoperative bleeding and less homologous blood usage. However, the effects on the hemostatic and fibrinolytic systems have remained in part unknown. RESULTS Indications of thrombin generation, platelet activation, and fibrinolytic activity were investigated in patients undergoing coronary artery bypass surgery. Two groups were perfused with cardiopulmonary bypass (CPB) circuits completely coated with surface-bound heparin: one group with low systemic heparin dose (activated clotting time [ACT] > 250 seconds; n = 17) and a second group having a full heparin dose (ACT > 480 seconds; n = 18). A third control group was perfused with ordinary uncoated circuits and full heparin dose (n = 17). The plasma level of thrombin-antithrombin complex and prothrombin fragment 1.2 increased in all groups during bypass, and somewhat more in both the heparin-coated groups toward the end of CPB, compared with the control group (P < .01). However, the increase during CPB was minimal compared with the major elevation observed 2 hours after surgery in all groups. Platelet release of beta-thromboglobulin increased in all groups (P < .01) during CPB and significantly more in the high-dose group compared with the other two groups (P = .03). Fibrinolytic activities were similar in all groups, and there were no indications of major consumption of coagulation factors. CONCLUSIONS Reduced systemic heparinization (ACT > 250 seconds) in patients having extracorporeal circulation with completely heparin-coated circuits did not lead to increased thrombogenicity. Thrombin formation remained within low ranges during CPB compared with patients receiving a full heparin dose and with the major elevations observed after surgery.