Allo - hematopoietic stem cell transplantation is currently the only way to cure myelodysplastic syndrome /acute leukemia . The existing experimental results showed that decitabine and 5-azacytidine up-regulated the expression of tumor Ags on leukemic blasts in vitro and expanded the numbers of immunomodulatory T regulatory cells in animal models. Reasoning that decitabine might selectively augment a graft versus leukemia effect, the investigators used decitabine administration after allogeneic stem cell transplantation to studied the immunologic sequelae.

Inclusion Criteria:

• Histologically confirmed AML in complete or partial remission or MDS using WHO classification undergoing alloHSCT
• High resolution typing HLA-matched related or unrelated donor. Donors may be mismatched at single antigen at HLA-A, -B or -DR locus plus possible single antigen mismatch at HLA-C according to institution guidelines. Two-antigen mismatch at a single locus is not allowed.
• Age ≥ 18
• creatinine < 1.5 times the institutional ULN or creatinine clearance (calculated by the Cockroft and Gault method) ≥ 30 mL/min
• bilirubin < 1.5 times the institutional ULN
• AST, ALT and alkaline phosphatase < 2.5 times the institutional ULN.

Exclusion Criteria:

• History of previous alloHSCT prior to the current alloHSCT.
• Persistent AML or MDS after alloHSCT.
• Positive serology for HIV.
• Pregnancy or nursing.
• Other cancers less than or equal to 2 years prior study entry except: basal cell carcinoma of the skin, squamous cell carcinoma of the skin, carcinoma in situ of the cervix, carcinoma in situ of the breast, prostate cancer stage T1a or T1b.
• Uncontrolled active infections requiring intravenous antibiotics. Clinically significant systemic illness (e.g. serious active infections or significant cardiac, pulmonary, hepatic or other organ dysfunction), which, in the judgment of the Principal or Associate Investigator would compromise the patient's ability to tolerate protocol therapy.
• Known or suspected hypersensitivity to decitabine.
• Patients may not be receiving any other investigational agents.
• General or specific changes in patient's condition that render the patient unacceptable for further treatment in judgment of the investigators.