Introduction: Donor lymphocyte infusion (DLI) is used to induce remission in patients who relapse after allogeneic stem cell transplantation (allo-HSCT). During the last decade, the hypomethylating agent Azacitidine has been used together with DLI for a synergistic graft-versusleukemia (GVL) effect. Here, we report results of DLI/ Azacitidine treatment from a retrospective singlecenter study. Methods: Fifty AML/MDS patients treated for relapse after allo-HSCT between 2001 and 2020 with DLI at the Department of Hematology, at Rigshospitalet, Copenhagen University Hospital were included for analyses. A subgroup of patients who obtained complete remission (CR) after reinduction chemotherapy, received DLI in combination with low-dose (32 mg/m2) Azacitidine. Results: Overall survival in all patients after DLI treatment was 59% at 2 years and 20% at 5 years. Relapsefree survival in patients in CR prior to DLI was 32% after 2 years and 7% after 5 years. In the DLI + low-dose- Azacitidine group, 5-year relapse-free survival was 40%. Conclusion: DLI remains an effective treatment in posttransplant relapse leaving one-fifth of patients? long-term survivors.Our results support the concomitant use of lowdose Azacitidine in the future use of DLI in order to enhance the GVL effect of donor lymphocytes. ? 2023 S. Karger AG, Basel.