Why is it necessary to check fecal occult blood when diagnosing patients with iron deficiency anemia? First, I would like to tell you about two personal experiences. When I was an intern, I once followed my hematology instructor on an outpatient clinic and treated a patient with iron deficiency anemia. After two weeks, I went to the general surgery internship rotation and saw this patient again. When I was an attending hematologist outpatient, a patient was referred to the emergency department once. The patient had nausea and dizziness after a meal at noon one day earlier, which was relieved after lying down to rest, but fainted after standing up, and was seen in the emergency department. The patient was given a routine fecal examination with tarry stools and positive occult blood. The family took the previous physical examination report, and the routine blood was microcytic hypochromic anemia, and the patient was from the south. The final diagnosis was thalassemia combined with upper gastrointestinal hemorrhage. Iron deficiency anemia is the most common type of anemia and is easy to diagnose and treat, but the most critical thing is to identify the cause of iron deficiency. Chronic bleeding from the GI tract is one of the common causes of iron deficiency, but it is often overlooked. The first patient had iron deficiency anemia due to chronic bleeding from gastric cancer, and the second patient had hemorrhagic anemia due to upper gastrointestinal hemorrhage, which was mistaken for iron deficiency anemia due to a combination of thalassemia minor. In both cases, the clues were found by fecal occult blood examination and the diagnosis was finally clarified. Therefore, fecal occult blood must be checked when making the clinical diagnosis of iron deficiency anemia, especially for male patients.