BACKGROUND Patients undergoing cardiac surgery with cardiopulmonary bypass (CPB) need many blood products due to deficiency of coagulation factors. Blood transfusion therapy in patients with excessive bleeding after CPB is generally empiric. We checked and studied the fibrinogen concentration and transfusion, as well as bleeding amount in the perioperative period. METHODS The study was approved by our institutional ethics committee. Thirty patients were studied. Blood samples were obtained at the induction of anesthesia (before CPB), at the end of CPB, at the end of operation, and on the next morning, or before the patient was given fresh frozen plasma in the intensive care unit. RESULTS For all cases, fibrinogen concentration and platelet concentration were lowest at the end of CPB. Fibrinogen concentration rose up to before CPB level on the next morning. The group in which fibrinogen concentration was less than 150 mg x dl(-1) at the end of CPB consumed more blood products than the group with fibrinogen concentration of over 150 mg x dl(-1). CONCLUSIONS Blood transfusion therapy based on fibrinogen concentration is needed to maintain adequacy of the perioperative blood transfusion and blood conservation in cardiac surgery.