The pathogenesis of Aplastic anemia is complex and involves an abnormal hematopoietic microenvironment, hematopoietic stem cell/progenitor cell deficiencies and immunity disorders. The etiological basis for marrow failure includes primary defects in or damage to the stem cell or the marrow microenvironment. The distinction between acquired and inherited disease may present a clinical challenge, Congenital or inherited causes of Aplastic anemia are responsible for at least 25% of children with this condition and for perhaps up to 10% of adults. Acquired causes of Aplastic anemia form (80%) of cases, include Idiopathic (> 80 %), Post-infectious 15% (particularly after hepatitis 9%, Epstein-Barr virus , human immune deficiency virus, parvovirus, and mycobacteria, Drug-induced and toxins (4%).