Inclusion Criteria:

- Ages 0-21

- AML in one of the following stages:

- Having preceding myelodysplasia (MDS)

- High Risk cytogenetics

- Requiring > 2 cycles chemotherpay to obtain complete remission

- Second of 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 as defined as either Philadelphia + ALL or MLL
rearrangement with slow early response, or hypodilpoidy, or end of induction M3
bone marrow, or end of induction M2 with M2-3 at Day 41.

- High Risk Second remission as defined as either bone marrow relapse < 36 months
from induction, or T-lineage relapse at any time, or very early isolated CNS
relapse or slow induction after relapse at any time

- Any third of 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
echocardioagram, or ejection fraction > 30% by radionucline angiogram.

- Adequate pulmonary function as defined by DLCO, FEV1/FVC > 60% by pulmonary
function tests. For children who are uncooperative for PFTs ans have no evidence
of dyspnea at rest of ecercise 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.