ACD is probably the most common anemia among hospitalized medical patients. It is variably defined by its clinical and, particularly, its laboratory manifestations. The most consistent features are low serum iron and normal or increased serum ferritin levels, reflecting normal or increased iron stores and distinguishing ACD from iron deficiency anemia. ACD often coexists with iron deficiency and the anemia of renal insufficiency. Most patients have an underlying infectious, inflammatory, or neoplastic disease, but as many as one quarter of patients do not. Several mechanisms have been proposed, the most significant of which are a block in reutilization of hemoglobin iron for red cell production and relative deficiency of erythropoietin, but the pathogenesis and mediators involved remain uncertain. The anemia itself seldom requires treatment and is ameliorated by successful treatment of the underlying disease.