In this study a control group of 523 patients, who were operated on in 1977 using extracorporeal circulation and whose drained blood had been rejected postoperatively, was compared to a group of 523 patients, operated on in 1978, whose blood was retransfused. The blood lost through substernal suction drainage was readministered during the first 12 h by means of a self-made retransfusion system that reemployed parts of the heart-lung machine. Age, sex, or type of operation were distributed evenly among the two groups. The mean postoperative blood loss amounted to 700 ml in the control group and to 720 in the retransfusion group. Accordingly, the necessary postoperative blood substitution was identical in both groups (retransfusion 2 units per patient compared to 2 units in the controls, 1 unit corresponding to 540 ml). In the retransfusion group 50% of the total mediastinal drainage blood was transfused. Thus the demand for donor blood was reduced by 50% in the retransfusion group (retransfusion group: 1 unit, control group 2 units per patient; P < 0.001). The number of operations that could be performed without donor blood increased from 27% (control) to 53% (retransfusion). Hemolytic and clotting studies of the mediastinal blood showed that it was defibrinized and contained markedly more serum hemoglobin, platelets, and clotting factors than the conserved blood. The considerable dilutional effect due to the retransfusion, however, had no influence on coagulation status or kidney function. There were no complications. With this system retransfusion of drainage blood is a simple and safe procedure. The reduction of donor blood transfused diminishes the patient's risk of hepatitis and means a financial advantage for the hospital.