The aim of this prospective randomized study was to compare the effects of the transfusion of unprocessed and cell saver-processed residual cardiopulmonary bypass (CPB) volume on haemostasis, complement activation, postoperative blood loss and transfusion requirements after elective cardiac surgery. Blood samples were taken at eight points in time, perioperatively. Haematological data, including haemoglobin, haematocrit and platelet counts as well as coagulation parameters, including activated partial thromboplastin time, prothrombin time, thrombin time, fibrinogen and the fibrinolytic parameter D-dimers, were measured from each blood sample. For the assessment of complement activation, the total complement CH50 was analysed. In addition, postoperative blood loss and transfusion requirements were measured during the first 24 hours, postoperatively. The results of the study showed impaired haemostasis after the transfusion of both unprocessed and processed CPB volume. No significant differences were found between the groups in the measured coagulation parameters. Nor was a significant difference found in the complement concentration. However, in patients transfused with unprocessed CPB volume, a significantly (p = 0.019) higher amount of blood loss was found, postoperatively. In the same group of patients, the number of units of allogeneic erythrocyte concentrate suspension transfused was also significantly (p = 0.023) higher during the first 24 hours, postoperatively, compared to the patients transfused with processed CPB blood. The number of units of fresh frozen plasma and platelet suspension transfused was not significantly different between the groups. In conclusion, processing CPB volume in combination with processing peroperative blood loss may result in reducing the volume of transfusion needed of allogeneic blood products.