Mismatched haploidentical donors will be identified for patients with severe aplastic anemia. These patients will undergo a preparative regimen of Fludarabine/Cyclophosphamide/TBI followed by haploidentical bone marrow transplantation. Post-transplant Cyclophosphamide will be administered on Days 3 & 4. Immunosuppression with Tacrolimus and MMF will begin on Day +5; MMF will be discontinued on Day +35 while Tacrolimus continues until Day +180. Investigators hypothesize that haploidentical transplantation with the above-mentioned preparative regimen will have a <30% graft failure rate. The one-sided exact Binomial test at 5% significance level will be used to test this hypothesis. The size of 20 patients provides the power of 92.5% for confirming the 30-day graft failure rate <30%.