Targeted immune therapy with gemtuzumab ozogamicin (Mylotarg) in combination with chemotherapy followed by allogeneic stem cell transplantation will be given to patients with high risk acute myelogenous leukemia (AML) or myelodysplastic syndrome (MDS).

Gemtuzumab ozogamicin in combination with busulfan and cyclophosphamide (immunochemotherapy) conditioning followed by allogeneic stem cell transplantation + anti-thymocyte globulin (unrelated donors only) in patients with high risk CD33+AML or MDS meeting eligibility criteria.

Inclusion Criteria:

• Disease Status:

  • History of AML Induction/Reinduction Failure
  • AML 1st Complete Remission (CR) with poor cytogenetics
  • AML 2nd CR with minimal residual disease (MRD)
  • AML 3rd CR
  • AML in refractory relapse but ≤25% bone marrow leukemia blasts
  • MDS with >6% bone marrow blasts at diagnosis
  • Secondary MDS with ≤5% bone marrow myeloblasts at diagnosis

Disease Immunophenotype:

• Disease must express a minimum of >/= 10% CD33+ for patients with AML

Organ Function:

• Adequate renal function, adequate liver function, adequate cardiac function, adequate pulmonary function Age: ≤25 years of age Donor: matched family donor, unrelated cord blood donor, unrelated adult donor

Exclusion Criteria:

• Patients with active central nervous system (CNS) AML/MDS disease at time of conditioning therapy
• Female patients who are pregnant
• Karnofsky <50% or Lansky <50% if 10 years or less
• Age >25 years
• Has received gemtuzumab in the previous 30 days or has not recovered from prior gemtuzumab therapy.