In cardiac surgery a reduction of homologous blood products is mainly achieved by autologous blood salvage. Unprocessed retransfusion of autologous blood may induce changes in hemostasis. We assessed changes of activated clotting times (ACT) after retransfusion of unprocessed (1) oxygenator blood and (2) shed mediastinal blood. In 41 patients undergoing cardiac surgery (37 revascularization and 4 valve procedures) with a mean cardiopulmonary bypass time of 116.3 min, ACT was measured at the following time points: pre- and postoperatively, and before and after retransfusion of oxygenator blood and shed mediastinal blood. A significant prolongation of ACT values was only seen between pre- to postoperative blood (p < 0.003). Retransfusion of oxygenator blood changed ACT from 120 +/- 25 vs 116 +/- 17 sec and retransfusion of shed mediastinal blood showed a mean ACT of 118 +/- 14 vs 115 +/- 20 sec from before to after transfusion. There was a significant correlation between the ACT value in the unprocessed oxygenator blood and the ACT value measured in the patient after retransfusion (r = 0.41, p < 0.02). 6 patients receiving over 1 litre of unprocessed autologous blood (either from the oxygenator or the mediastinum) in less than 60 minutes showed a significant increase of ACT value after transfusion (p < 0.05). Hence, under normal conditions, retransfusion of unprocessed autologous blood will not significantly alter hemostasis measured by the ACT test in patients undergoing cardiac surgery.