Today the accumulating mass of RDW literature supports the conclusion that RDW is a valid and major predictor of health outcomes. Previously RDW parameter was used only as indicator for red blood cells anisocytosis for differential diagnosis of microcytic hypochromic anemia (iron deficiency versus thalassemia minor) and macrocytic anemia (megaloblastic anemia) . Today the vast study of different chronic diseases especially of cardiac, renal, diabet and cancer showed that RDW can be used as a potent independent factor for prognosis and monitoring chronic diseases. It is now approved that not only iron and vitamins deficiency affect RDW level but also different pathologic conditions such as telomere length, stress, neuroendocrine abnormalities, hypoxemia, and malnutrition and … influences RDW level.