In a randomised prospective study the efficacy of autotransfusion was investigated in two groups of 25 patients, a study group in which autologous blood was collected from the mediastinal tubes and retransfused, and a second, control group, in which only stored blood was used. In the study group, a reduction of 50% in the amount of stored blood required was observed. However, in two out of 25 patients the transfusion system could not be used due to clot formation in one of its components. From the total bloodloss per patient about 25% became available for autotransfusion. No significant differences between the two groups were found for hemoglobine, hematocrite, white blood count, platelets and fibrinogen level. Coagulation studies of the drain blood indicate that an active process of mediastinal clotting, followed by fibrinolysis occurs during the losing and collecting. The blood available for retransfusion contained a considerable amount of small-sized debris. It is concluded that autotransfusion of drain blood is not to be recommended for routine use.