THE HEMORRHAGIC SYNDROME in patients operated upon for correction of cardiac defects with the aid of a pump oxygenator constitutes a significant problem. Study of this problem in over 60 patients has furnished information indicating that, as a rule, more than one factor is responsible for postoperative bleeding tendencies occasionally noted in these patients. In a number of instances, preexisting defects can be demonstrated in the cellular and plasmatic components of blood. These abnormalities may disturb the normal coagulation mechanism. During and after cardiac bypass these changes may become magnified and severe enough to result in hemorrhagic diathesis. Methods In 60 patients the fibrinogen concentration, plasminogen concentration, platelet count and thrombin time, preoperative hemoglobin, Lee White clotting time, one-stage prothrombin time (Quick), and platelet count were determined as described elsewhere. Preexisting Defects Prolongation of Lee White clotting time in a number of patients appeared to occur more frequently in cyanotic