Severe aplastic anemia (SAA), which is considered to be an immune-mediated destruction of bone marrow stem cells with pancytopenia and hypoplasia, can be successfully treated with immunosuppressive therapy or hematopoietic stem cell transplantation (HSCT). Between January 2009 and December 2012, thirteen patients diagnosed with SAA were treated with tacrolimus plus rabbit antithymocyte globulin (ATG)-based immunosuppressive therapy (IST). The outcomes were then compared with our previous data for twenty-four patients administered with cyclosporine (CsA) plus rabbit ATG-based IST. All 37 cases accepted methylpredenisolone and recombinant human granulocyte colony-stimulating factor (rhG-CSF) from the first day that rabbit ATG was initiated. A total of 7 (54%) of the 13 patients in the tacrolimus group and 10 (42%) of the 24 cases in the ATG + CsA group achieved the criteria for complete response (CR); the partial response (PR) rate was 31% in the tacrolimus group and 33% in the ATG + CsA group. The median follow-up duration of the tacrolimus group and ATG + CsA group patients was 28 months and 27 months, respectively. Two patients in the tacrolimus group who were red blood cell- and platelet transfusion-dependent died, one of sepsis and the other of cerebral hemorrhage, whereas one patient died from serious infection on the 5th day after ATG was initiated in the ATG + CsA group. No clonal transformation to paroxysmal nocturnal hemoglobinuria (PNH) was observed in either group. Our data provide a possibility of using tacrolimus as part of an IST regimen for SAA in adults who have no opportunity of HSCT from human leukocyte antigen (HLA)-matched sibling donors. (C) 2014 Elsevier Inc. All rights reserved.