BACKGROUND Introduction of completely heparin-coated cardiopulmonary bypass (CPB) circuits combined with reduced systemic anticoagulation has been shown to reduce postoperative bleeding and requirements for allogeneic transfusions after cardiac surgery. However, some uncertainty exists whether this effect is due to the reduced amount of heparin or to the heparinized surface itself. Therefore, a retrospective study was undertaken, comparing two different anticoagulation protocols applied to coronary artery bypass patients treated with identical heparin-coated CPB equipment. METHODS Over a 12 month period all coronary artery bypass patients operated with extracorporeal circulation were subjected to a Duraflo II heparin-coated circuit (Baxter Healthcare Corp, Bentley Laboratories Division, Irvine, Calif) and full heparin dose (activated clotting time [ACT] > 480 seconds; Group F, n = 651). Over the next 24 months, all coronary patients who were treated with an identical circuit combined with reduced systemic heparinization (ACT > 250 seconds) were included in Group R (n = 675). Except for the different anticoagulation protocols, all treatment regimens before, during, and after the operation remained unchanged throughout the study period. RESULTS There were no statistically significant differences in any major demographic or operative parameters. In Group R, the postoperative bleeding was mean 665 +/- 257 ml versus 757 +/- 367 ml in Group F (p < 0.0001), and the perioperative decrease in hemoglobin concentration was significantly lower in Group R (22 +/- 1.2 gm/L versus 25 +/- 1.3 gm/L, p < 0.0001). The time for postoperative ventilatory support was shorter in Group R (1.7 +/- 1.3 hours versus 1.9 +/- 1.1 hours in Group F, p = 0.0006), and the incidence of new episodes of atrial fibrillation after the operation was lower (26.4% in Group R versus 32.8% in Group F, p = 0.01). There were no significant differences in the incidences of perioperative myocardial infarction, stroke, transient neurological disturbances, physical rehabilitation, or mortality. No technical or coagulation problems were recorded in either group. CONCLUSIONS The use of Duraflo II coated circuits for CPB combined with reduced anticoagulation decrease postoperative bleeding and hemoglobin loss compared with full heparin dose treatment. In addition, the intubation time was shorter and the incidence of postoperative atrial fibrillation was lower in the patients treated with low heparin doses.