Background: To determine whether 500,000 KIU aprotinin is effective to reduce blood loss in coronary artery bypass grafting (CABG) and to evaluate the effects of this regimen on hematologic parameters. Methods: Forty-four patients scheduled for primary CABG were randomly assigned to the aprotinin (n = 24) or control group (n = 20). In aprotinin group, aprotinin was administered in two equal doses (before skin incision and added to the pump prime). Ventilation time, intensive care unit stay, mediastinal tube drainage, hospitalization, transfusion requirements, and postoperative morbidities and mortality were noted. Hematologic markers of fibrinolytic activity and complement activation were also measured pre- and postoperatively. Results: Although less mediastinal drainage occurred in aprotinin group, the difference was not statistically significant. Other postoperative variables like transfusion requirements, morbidities, and mortality were also found to be similar between groups. Among hematologic parameters, only postoperative levels of alpha 2-antiplasmin and plasminogen activator inhibitor-1 were significantly higher in aprotinin group. Conclusions: Although plasmin inhibitors begin to rise at this very low aprotinin dosage, it is not advisable to use this aprotinin regimen in CABG patients.