The present project is a multicenter, phase II trial which aims at evaluating if the administration of azacytidine (Vidaza®) combined to donor lymphocyte infusion (DLI) could improve the response rate to DLI in the population of patients with relapsed acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) after allogeneic hematopoietic stem cell transplantation.