Azacytidine and Lymphocytes in Relapse of AML or MDS After Allogeneic Stem Cell Transplantation.

Inclusion Criteria: 1. Patients: * Age ≥ 18 years * Be able to understand and sign informed consent * Fertile patients must use a reliable contraception method 2. Disease status at transplantation: * AML in first or subsequent complete remission (\< 5% marrow blasts) * MDS with less than 10% marrow blasts at the time of transplantation 3. Transplantation: * Allogeneic transplantation using a sibling or unrelated donor with matching in 10/10 alleles (HLA-A, B, C, DRB1, DQB1) or maximum of one allele or one antigen or 1 antigen + 1 allele or 1 antigen + 1 DQB1 antigen or 2 alleles mismatches. * Myeloablative or reduced-intensity conditioning * Second transplantation is allowed * Donor is willing to donate lymphocytes 4. Clinical situation: * Cytological relapse after allo-SCT defined as the recurrence of more than 5% blasts on bone marrow aspiration (AML) or evidence of MDS * Immunophenotypic relapse defined as the recurrence of an abnormal phenotype on flow cytometry in bone marrow aspirate (only in case of a specific phenotype). * Cytogenetic or molecular relapse defined as the persistence or recurrence of a cytogenetic abnormality or molecular marker in bone marrow aspiration or peripheral blood. WT1 expression is not considered as reliable marker for relapse in this protocol but FLT3-ITD, NPM1, CEBPA, or translocation-specific markers (such as MLL-PTD, AML-ETO, CBFB-MYH11) are. 5. Immunosuppressive therapy should have been stopped before inclusion. Exclusion Criteria: * More than 30% marrow blasts at the time of inclusion * Extramedullary relapse including CNS involvement * ECOG Performance status \> 2 * Active acute grade II-IV GvHD at the time of inclusion * Active chronic GvHD requiring systemic therapy at the time of inclusion * Uncontrolled infection * HIV positive * Acute or chronic heart failure (NYHA class III or IV) or symptomatic ischemic heart disease or ejection fraction \< 35% or uncontrolled arrhythmia * Severe liver failure (total bilirubin \> 3 mg/dL, SGPT \> 4 X upper normal limit) * Severe pulmonary failure (corrected DLCo \< 35%) * Terminal renal failure requiring dialysis * Severe neurological or psychiatric disorders * Concurrent investigational drug. * Other treatment for relapse, except for hydroxyurea but it should be stopped before inclusion in the study. * Female who is pregnant or breastfeeding