Phase I, Dose Escalation Study of Decitabine

Inclusion Criteria: * Age: greater than 1 and less than 31 years of age; * Diagnosis: history of ALL, AML or MDS, currently in a complete remission (CR) following allo-HSCT (bone marrow leukemic blasts less than 5% by morphology), with high risk features including: * Status post allogeneic HSCT * GVHD prophylaxis: * Karnofsky or Lansky performance scores more than 50%. Karnofsky scores will be used for patients \> 16 years of age and Lansky scores for patients ≤ 16 years of age; * Platelet count ≥ 50,000 (untransfused); * Absolute neutrophil count ≥ 1000; and; * Hemoglobin ≥ 8 g/dL (un-transfused); Exclusion Criteria: * Progressive disease; * Philadelphia chromosome positive ALL (these patients receive tyrosine kinase inhibitor posttransplant); * Known hypersensitivity to any components of decitabine; * Uncontrolled grade 3-4 graft versus host disease; * Uncontrolled infection; * Serum creatinine \> 2 mg/dL or glomerular filtration rate (GFR) less than 60 mL/min/1.73m2 ; * Alanine Aminotransferase (ALT) greater than 3 times normal or serum total bilirubin greater than 2 mg/dL;