Inclusion Criteria:

• Age: Patients must be <30 years of age.
• Disease Status: ALL in relapse, induction failure, CR3, or CR3P (Part A ONLY); AML in relapse, induction failure, CR3, or CR3P (Part A ONLY); ALL in CR3 or CR3P (Part A and Part B); AML in CR3 or CR3P (Part A and BONLY); CR3/CR3P must be documented by bone marrow and CNS assessment within 14 days of initiation of the pre-transplant conditioning regimen.
• Creatinine clearance >40 ml/min/m2 or >60 ml/min/1.73 m2 or an equivalent radioisotope glomerular filtration rate (GFR) as determined by the institutional normal range or serum creatinine based on age
• Adequate liver function defined as: Total bilirubin <2.5 mg/dl l, or SGOT (AST) or SGPT (ALT) <5 x upper limit of normal
• Adequate cardiac function defined as: Shortening fraction >27% by echocardiogram, or Ejection fraction of >50% by radionuclide angiogram or echocardiogram.
• Adequate pulmonary function defined as: Corrected DLCO >60% by pulmonary function test; For children who are uncooperative, no evidence of dyspnea at rest, no exercise intolerance, and a pulse oximetry >94% on room air.
• Performance Status: For patients age 1-16 years, Lansky score of >60; For patients > 16 years, Karnofsky score of >60.
• Patients must have received a minimum of one round of re-induction and one round of consolidation chemotherapy after relapse #2

Exclusion Criteria:

• Patients with prior myeloablative allogeneic stem cell transplantation and /or TBI.
• Females who are pregnant (positive HCG) or lactating.
• Karnofsky <60% or Lansky <60% if less than 16 years of age
• Age >30 years of age
• Any patient with uncontrolled infection prior to study entry
• Patients with evidence of active disease.
• Patients with Down syndrome are excluded