In normal controls and patients with various blood diseases, biopsies of the duodenal mucosa were performed for an investigation of histopathological changes and their relation to intestinal iron absorption. Followings are the results. 1. In essential hypochromic anemia, leukemia, and anemia associated with cancer, inflammatory changes were observed, and in a case of acute myelogenous leukemia, infiltration of immature myelogenous cells was observed. 2. Little inflammatory changes were noted in aplastic anemia, suggesting that histological changes in the intestinal mucosa are not related at least to the genesis of this disease. 3. In a case of hookwarm anemia, remarkable inflammatory changes were noted, suggesting the presence of disturbances in the intestinal iron absorption as one of the causes of this anemia. 4. No significant histological changes were observed in myxedema, Mikulicz syndrome, purpura simplex, and pernicious anemia. 5. The above investigations suggest that inflammatory changes of the intestinal mucosa generally have some influences on the iron absorption.