['One cause of diffuse bleeding after cardiac operations may be a low plasma concentration of coagulation Factor XIII, which is essential for coagulation but is not covered by standard coagulation monitoring.', 'In a prospective, randomized, double blinded study, 2500 units, 1250 units, and a placebo were administered in groups of 25 patients each, immediately after administration of protamine. Postoperative amount of blood loss and blood transfusion was recorded.', 'Patients were not statistically different with respect to the course of plasma levels of Factor XIII until administration of the study drug. In all groups Factor XIII fell from preoperative normal values to subnormal values after extracorporeal circulation. After administration of the study drug, Factor XIII increased to 71 %, 85 %, 103 % in the placebo, 1250 units, and 2500 units group, respectively, and these differences were statistically significant ( p < 0.05). Postoperative blood loss was lowest in the 2500 units group and highest in the placebo group, however this was not significantly different. There was also no significant difference in the amount of blood transfusion. After differentiating all patients according to their post medication Factor XIII level into two groups with levels of < 70 % and > or = 70 %, postoperative blood loss was found to be significantly higher in the < 70 % group as was the amount of blood transfusions.'] Does coagulation factor XIII reduce postoperative bleeding after coronary surgery with extracorporeal circulation?