OBJECTIVE: The study was designed to determine differences in blood loss and transfusion associated with a minimized cardiopulmonary bypass circuit versus a standard bypass circuit. METHODS: From February 2005 through April 2006, 199 patients were randomized to undergo coronary artery bypass grafting with a standard cardiopulmonary bypass circuit (Medtronic, Inc., Minneapolis, Minn) or a minimized bypass circuit, the Medtronic Resting Heart Circuit. Laboratory perimeters (hemoglobin and platelet count), were measured at baseline, after initiation of cardiopulmonary bypass, and on intensive care unit admission. Lowest values recorded were noted. Blood administration was controlled by study-specific protocol orders, (transfusion for hemoglobin <8mg%). Patient demographic data were retrieved from the Society of Thoracic Surgeons database. Blood product administration was recorded during hospital admission, and chest tube drainage as total output collected from operating room to discontinuation. Continuous variables were tested with a Wilcoxin rank test, and categoric variables with X(2) and Fisher's exact tests. RESULTS: Hematocrit, equivalent at baseline, was higher in minimized circuit cohort at lowest point during cariopulmonary bypass (31.5% +/- 3.9% vs. 25.5% +/- 3.7%), after protamine (31.6% +/- 3.9% vs 29.2% +/- 3.7%), and on intensive care unit arrival (35.2% +/- 4.1% vs 31.8% +/- 3.5%, P < .001). Similarly, platelet count was higher in minimized circuit group on intensive care unit arrival, as was lowest platelet count recorded (170 x 10(3) +/- 48 cells/mm(3) vs 107 x 10(3) +/- 28 cells/mm(3), P < .0001). Time to extubation was shorter in minimized circuit group (848 +/- 737 minutes vs. 526 +/- 282 minutes, (P < .01), and total chest tube drainage was lower (1124 +/- 647 mL vs. 506 +/- 214 mL, P < .01). Fewer red blood cells (148 vs 19 units) were given in minimized circuit group (P < .0001). CONCLUSIONS: A minimized cardiopulmonary bypass circuit provides less hemodilution, platelet consumption, chest tube output and lower post-operative blood loss than standard cardiopulmonary bypass. Red blood cell usage was also less. All differences are advantageous.