The minimal cardiopulmonary bypass (mini-CPB) circuit, a closed system with neither cardiotomy suction nor an open venous reservoir and thus no air–blood interface, reportedly reduces blood loss and inflammatory reactions associated with coronary bypass surgery. We evaluated the inflammatory reactions in patients in whom coronary bypass operations were performed with conventional CPB or mini-CPB (n = 15 each). Interleukin (IL)-6, IL-8, and neutrophil elastase levels; the neutrophil count; and the C-reactive protein value were measured before and immediately after surgery and on postoperative days 1 and 2. In addition, intraoperative blood loss and the transfusion volume were evaluated in these groups. Neutrophil elastase levels were lower in the mini-CPB group than in the conventional group on postoperative days 1 (127 ± 52 vs. 240 ± 100 µg/l, P = 0.013) and 2 (107 ± 17 vs. 170 ± 45 µ/l, P = 0.0001), as was the IL-8 level on postoperative day 1 (8.3 ± 6.4 vs. 19 ± 11 pg/ml, P = 0.016). The intraoperative blood loss and transfusion volumes were significantly lower in the mini-CPB group than in the conventional group (510 ± 244 vs. 1046 ± 966 ml, P = 0.012, and 691 ± 427 vs. 1416 ± 918 ml, P = 0.0033). Thus, mini-CPB appears to attenuate neutrophil activation and cytokine release after coronary bypass surgery and, in addition, has some beneficial effects on blood conservation.