"Identification error between patient and blood product" is the main cause of ABO-incompatible blood transfusion, but "Phlebotomy error" also has serious consequences. In order to prevent ABO-incompatible transfusion, it is important to establish a management system of blood transfusion in the hospital, including a hospital transfusion committee and a responsible medical doctor. In addition, in large hospitals routinely carrying out a considerable number of blood transfusions, it is important to employ specialists in blood banking. More than 50 ml of ABO-incompatible blood transfusion (major ABO mismatch) causes a severe acute hemolytic reaction. Because there is little residual plasma in leukocyte-reduced red cell concentrate (RCC-LR), acute hemolysis is not detected on minor ABO mismatch blood transfusion. In the case of emergent blood transfusion, concerning the risk of acute hemolytic reaction, type-O RCC-LR blood transfusion is safer than ABO-identical RCC-LR when the blood of the patient is tested only once. When red cell antibody screening is not performed, there is a risk of hemolysis due to incompatible blood transfusion irrespective of the ABO blood group system, including a delayed hemolytic transfusion reaction.