Maintenance Low Dose 5'-Azacitidine Post T Cell Depleted Allogeneic Stem Cell Transplantation for Patients With Myelodysplastic Syndrome and Acute Myelogenous Leukemia With High Risk for Post-Transplant Relapse

Inclusion Criteria: Patients who have undergone T cell depleted allogeneic hematopoietic stem cell transplantation at MSKCC for: * De novo myelodysplastic syndromes (MDS): IPSS-1 with poor risk cytogenetics or higher IPSS. * Acute myelogenous leukemia (AML) in first remission that required more than 1 cycle of treatment to achieve remission or with the following cytogenetic abnormalities: FLT3 mutation, deletion/monosomy of chromosome 5 or 7, MLL gene rearrangement, or more than or equal to 3 cytogenetics abnormalities. Also patients in second or greater remission. * Patients with Secondary MDS/AML. * Patients will be considered eligible for the study if after transplant they achieved hematologic (\<5% blasts) and cytogenetic remission. * Patients will be eligible to enter the study between 60-120 days post transplant. * Age: pediatrics and adults patients - 1 year old-75 years old. * Karnofsky performance status \>=60% for patients \>16yo and Lansky performance status \>=60% for patients ≤16yo * Stable blood counts (ANC\>1000/uL, Hb\>8gr/dL, Plt\>50,000/ uL) not supported by transfusions. * Renal: Serum creatinine \<1.5 ULN * Hepatic: \<3xULN ALT and \<1.5 total serum bilirubin, unless there is congenital benign hyperbilirubinemia. * Cardiac: Adequate cardiac function measured by LVEF\>50%. If asymptomatic, pretransplant echocardiogram is adequate. If symptomatic, echocardiogram needs to be repeated. * Each patient must be willing to participate as a research subject and must sign an informed consent form. Exclusion Criteria: Patients will be excluded from the trial if at time of enrollment: * Active uncontrolled bacterial, fungal or viral infection. * Evidence of uncontrolled graft-versus-host disease. * Pulmonary: new onset hypoxia * Known or suspected hypersensitivity to 5'-azacitadine or mannitol. * Evidence of residual disease either by increased blasts count (\>5%) or persistence of previous known cytogenetics abnormalities. * Peripheral blood neutrophil chimerism: less than 95% donor.