Inclusion Criteria:

          -  Ages 0-21

          -  AML in one of the following stages:

               -  Having preceding myelodysplasia (MDS)

               -  High Risk cytogenetics

               -  Requiring > 2 cycles chemotherapy to obtain complete remission

               -  High allelic ratio FLT3/ITD+,

               -  Standard risk cytogenetics with positive MRD at end of Induction

               -  Second or greater CR

               -  First relapse with < 25% blasts in bone marrow

               -  With therapy-related AML whose prior malignancy has been in remission for at
                  least 12 months

          -  ALL in one of the following stages:

          -  High risk first remission, defined as:

               -  Ph+ ALL; or,

               -  MLL rearrangement with slow early response [defined as having M2 (5-25% blasts)
                  or M3 (>25% blasts on bone marrow examination on Day 14 of induction therapy)];
                  or,

               -  Hypodiploidy (< 44 chromosomes or DNA index < 0.81); or,

               -  End of induction M3 bone marrow; or,

               -  End of induction M2 marrow or MRD>1% with M2-3 marrow or MRD>1% at Day 42.

               -  High-risk infant ALL defined as age <6 months at diagnosis with MLL (11q23)
                  translocation.

          -  High risk second remission, defined as:

               -  Bone marrow relapse < 36 months from induction; or >36 mths if a matched sibling
                  donor is available

               -  T-lineage relapse at any time; or,

               -  Very early isolated CNS relapse (<18 months from diagnosis); or,

               -  Slow reinduction (M2-3 at Day 28) after relapse at any time.

          -  Any third or subsequent CR.

          -  Biphenotypic or undifferentiated leukemia in any CR or if in first relapse must have <
             25% blasts in bone marrow

          -  MDS at any stage; prior therapies allowed

          -  CML in chronic or accelerated phase; prior therapies allowed

          -  Patient also must have the following organ requirements:

               -  Adequate renal function defined as serum creatinine <2x normal, or creatinine
                  clearance > 40 ml/min/m^2 or 70 ml/min.

               -  Adequate liver function as defined by total bilirubin less than or equal to 2
                  times normal and AST and ALT less than or equal to 4 times normal.

               -  Adequate cardiac function as defined by: shortening fraction > 24% by
                  echocardiogram, or ejection fraction > 30% by radionuclide angiogram.

               -  Adequate pulmonary function as defined by DLCO, FEV1/FVC > 60% by pulmonary
                  function tests. For children who are uncooperative for PFTs and have no evidence
                  of dyspnea at rest or exercise intolerance, pulse oximetry > 94% on room air is
                  considered acceptable, with a normal chest xray.

               -  Adequate venous access; a double lumen central vascular access device or its
                  equivalent and an additional PICC line will be required for all patients.

          -  Women of childbearing potential and sexually active males should use effective
             contraception while on study.

        Exclusion Criteria:

          -  Inability to give informed consent or assent

          -  Inability to obtain a suitable donor

          -  Patient who is HIV-positive

          -  Patient who has active Hepatitis B

          -  Patient who is pregnant

          -  Patient who is otherwise considered unsuitable for transplant at the discretion of the
             principal investigator.