Anemia occurs commonly and is associated with adverse outcomes in the elderly. In approximately one third of anemia cases in patients over the age of 65 years, the cause of anemia is not readily apparent (unexplained anemia or UA). Of the various causes of anemia in young adults, overt hemolysis (either acute or chronic) is very uncommon. However, we speculate that older persons tend to develop a low grade hemolytic process which significantly reduces RBC survival, and when this is not adequately countered by increased bone marrow RBC production, anemia (UA) is the consequence. This hypothesis is supported by a few clinical observations. For example, red cells in patients with UA are generally not small and when the peripheral blood smear is examined microscopically, anisocytosis (varying cell size) is observed, as is typical in patients with hemolytic anemia. Similarly, UA is frequently associated with an elevated red cell distribution width (RDW) on electronic measurement. Furthermore, serum erythropoietin levels gradually rise with advancing age (1), and this would be consistent either with a smoldering hemolytic process or a decreased responsiveness to erythropoietin. To address the hypothesis that UA is due, at least in part, to shortened RBC survival, we propose to directly measure red blood cell survival and to correlate this with aspects of red blood cell physiology that may increase susceptibility to the hemolytic process.