SEVERAL studies of the prevention and treatment of excessive bleeding in extracorporeal circulation have been published.1-17 While inadequate surgical hemostasis11,12,15 and incomplete heparin neutralization7,8,15-17 are the most common causes of hemorrhage, changes in the platelets and plasma coagulation factors do occur, but they only occasionally contribute to blood loss.2-8,10-14,16,17 For the past five years, all candidates for open heart surgery at the Toronto General Hospital have routinely had preoperative studies of hemostatic function. During the past year similar tests have also been performed at the completion of perfusion. This review was undertaken in order to determine the cause of excessive blood loss and to assess the ability of the preoperative hemostatic tests to predict operative hemorrhage. While major bleeding is now an infrequent complication after cardiopulmonary bypass at this hospital, a recent survey18 suggests that bleeding is still a major problem with some teams of