To determine the hemostatic effets of aprotinin in coronary artery bypass surgery, a prospective study was performed. Thirty four patients (group A) who received aprotinin (2×10⁶ unit) in the cardiopulmonary circuit were compared with 31 control patients (group C) who did not. Activated clotting time was measured in group A for 60 minutes during cardiopulmonary bypass, but no significant difference was observed in the platelet counts of the two groups. Intraoperative blood loss was 366ml in group A compared with 514ml in group C, and postoperative blood loss was 354ml versus 570ml, respectively (p<0.05). Total blood loss was significantly reduced in group A by 34% compared with group C (720ml versus 1, 084ml, p<0.05). There was no difference in incidence of perioperative myocardial damage. It seems necessary to investigate the complications of the hemostatic effects of aprotinin and it effects on graft patency.