To be eligible to participate in the randomized trial, an individual must meet all thewing criteria:1. Provision of signed and dated informed consent form for the randomized trial byd/or legal guardian.2. Age ≤25 years old at time of randomized trial consent.3. Confirmed diagnosis of idiopathic SAA, defined as:1. Bone marrow cellularity <25%, or <30% hematopoietic cells.2. Two of three of the following (in peripheral blood): neutrophils <0.5 x 10^9/L,<20 x 10^9/L, absolute reticulocyte count <60 x 10^9/L or hemoglobin <8g/dL.. No suitable fully matched related donor available (minimum 6/6 match for HLA-A and Bdiate or high resolution and DRB1 at high resolution using DNA basedyping).5. At least 2 unrelated donors noted on NMDP search who are well matched (9/10 or 10/10HLA-A, B, C, DRB1, and DQB1 using high resolution).6. In the treating physician's opinion, no obvious contraindications precluding them fromBMT or IST.usion Criteria:1. Presence of Inherited bone marrow failure syndromes (IBMFS). The diagnosis of Fanconiust be excluded by diepoxybutane (DEB) or equivalent testing on peripheralblood or marrow. Telomere length testing should be sent on all patients to excludeDyskeratosis Congenita (DC), but if results are delayed or unavailable and there areDC, patients may enroll. If patients have clinicalharacteristics suspicious for Shwachman-Diamond syndrome, this disorder should beuded by pancreatic isoamylase testing or gene mutation analysis (note: pancreaticylase testing is not useful in children <3). Other testing per center may bed to exclude IBMFS.2. Clonal cytogenetic abnormalities or Fluorescence In-Situ Hybridization (FISH) patternwith pre- myelodysplastic syndrome (pre-MDS) or MDS on marrow examination.3. Known severe allergy to ATG.. Prior allogeneic or autologous stem cell transplant.5. Prior solid organ transplant.6. Infection with human immunodeficiency virus (HIV).7. Active Hepatitis B or C. This only needs to be excluded in patients where there isuspicion of hepatitis (e.g., elevated LFTs).8. Female patients who are pregnant or breast-feeding.9. Prior malignancies except resected basal cell carcinoma or treated cervical carcinomau.10. Disease modifying treatment prior to study enrollment, including but not limited touse of androgens, eltrombopag, romiplostim, or immune suppression. Note: Supportiveures such as G-CSF, blood transfusion support and antibiotics are allowable