A 67-year-old man was scheduled for resection of thoracic intramedullary tumor. His blood type was AB. Unexpectedly rapid and massive hemorrhage occurred during the operation. We infused a large amount of crystalloid, colloid, and 5% albumin. But he went into circulatory collapse before we could get type AB red blood cell (RBC) and hemoglobin was reduced to 3.8 g x dl(-1). We decided to transfuse 4 units of ABO incompatible type O RBC. Later, type AB RBC and other blood components became available and the patient recovered. We conclude that transfusion of ABO incompatible blood in patient with critical hemorrhage should be encouraged to save life.