Bleeding during the first 24 hours following cardiac surgery using cardio-pulmonary bypass (CPB) is a serious complication. Attempts to modify the degree of postoperative bleeding with pharmacologic therapy have met with limited success. Tranexamic acid, a potent inhibitor of plasminogen, may decrease the amount of mediastinal bleeding following surgery utilizing CPB. We assigned 30 patients who were about to undergo cardiac surgery to a double-blind randomized trial. The treatment group received tranexamic acid, given intravenously as 10 mg/kg over 30 minutes, at the time of skin incision, followed by a 1 mg/kg/hr infusion for 12 hours. The control group received a placebo (saline) of equal volume. Measurements of shed mediastinal blood and transfused homologous blood were made at 6, 12, and 24 hours postoperatively. Mediastinal blood loss in the treatment and control groups was 382 mL versus 594 mL at 6 hours (P = 0.08), 502 mL versus 848 mL at 12 hours (P = 0.04), and 711 mL versus 1160 mL at 24 hours (P = 0.02). The mean transfusion volumes after 24 hours were 356 mL in the treatment group and 528 mL in the placebo group (P = NS). Prophylactic tranexamic acid infusion decreases mediastinal blood loss following cardiopulmonary assisted cardiac surgery.