Idiopathic aplastic anemia (AA) is an autoimmune disease caused by T cells. An increase in the percentage of glycosylphosphatidylinositol-anchored protein-deficient cells and the presence of HLA allele-lacking leukocytes due to 6pUPD provide indirect evidence that T cells contribute to the pathophysiology of AA. Recent studies have revealed the presence of somatic mutations in MDS and/or AML candidate genes in one third of AA patients. Current treatment topics include the efficacy of eltrombopag for AA found to be refractory to immunosuppressive therapy as well as for newly diagnosed AA when administered in combination with ATG and cyclosporine. Furthermore, improved outcomes of allogeneic bone marrow transplantation from unrelated donors using reduced-intensity conditioning regimens have been obtained with eltrombopag. Fludarabine-based regimens are now the mainstream approach for preconditioning and have lowered the transplant-related mortality rate. However, new problems such as mixed chimerism and secondary graft failure have arisen. Attempts to prevent GVHD more efficiently by including ATG and alemtuzumab in the preconditioning regimen are being investigated.