Despite improvements in outcomes after Hematopoietic Cell Transplantation (HCT) for Acute Myeloid Leukemia (AML) and Myelodysplastic Syndrome (MDS), the risk of relapse remains high and is the most common cause of mortality after HCT. Moreover, treatment options for relapse after HCT are limited. Strategies to reduce relapse with maintenance therapy in patients who are at high risk are needed to improve survival. 5-aza is a hypomethylating agent that has shown immune modulating properties that may enhance the graft-versus-leukemia (GVL) effect, including upregulation of tumor-associated antigen and costimulatory molecule expression. Moreover, 5-aza has properties that suggest protection against graft-versus-host disease (GVHD) as well. Preliminary data shows that it is well tolerated and effective in clinical use for the treatment of AML or MDS relapse after HCT, as well as for maintenance therapy. This study will evaluate the use of 5-aza for maintenance after HCT in patients with AML or MDS with risk factors that are associated with a high risk for relapse.