Preoperative blood transfusion is sometimes controversial. We describe a case of a 43-year-old woman who developed sudden pulmonary congestion during surgery despite a small amount of intravenous crystalloid administration. She had no allergic disorders. Preoperative examination revealed that her hemoglobin was 5.6 g/dl, and she was diagnosed as folate-deficiency anemia although she never felt any symptoms before. Therefore, blood transfusion was performed and her hemoglobin increased to 9.4 g/dl. Amid surgery, airway pressure increased suddenly and pulsed oxygen saturation dropped. Chest roentgenogram revealed that the lungs were congested and her heart was markedly enlarged indicating the state of circulatory overload. We suggest that preoperative transfusion in a patient with chronic anemia should be carefully considered, and strongly recommend the confirmation with chest roentgenogram.