Cardiac surgery frequently requires blood transfusion. The use of transfusion should be restricted due to side effects. Blood transfusion via the cardiopulmonary bypass (CPB) circuit is easy and allows a fast transfusion. The administration of packed red cells is relatively frequent because of the CPB-induced hemodilution and of the higher rate of postoperative complications when the haematocrit during CPB decreases below 20%. This transfusion of packed red cells does not seem to be associated with complications during CPB. Platelet transfusion during bypass is illogical because of the destruction of platelets during CPB and must be avoided. Fresh frozen plasma transfusion during CPB is seldom indicated but is possible. It could reverse heparin resistance.