Purpose: The prevention of relapse is the major therapeutic challenge in older patients with acute myeloid leukemia (AML) who have obtained a complete remission on intensive chemotherapy. There is no established post-remission treatment for prevention of relapse in this setting, except allo HSCT. This randomized phase III study (HOVON97) in older patients (≥ 60 years) with AML or MDS-RAEB in CR/CRi after at least 2 cycles of intensive chemotherapy assessed the value of azacitidine as post remission therapy (in comparison to observation) with respect to the disease free survival (primary endpoint) and overall survival (secondary endpoint). Patients and methods: In total 117 patients were randomly 1:1 assigned to either observation (control group ) or azacitidine maintenance (aza group), 50 mg/m2, s.c., day 1-5, q 4 weeks, until relapse for a maximum of 12 cycles. The study was powered to detect improvement in disease free survival (DFS) after 1 year from 40% to 60% (power 80%, two-sided test at 5% significance). The current pre-final evaluation after study closure is based on the first 109 patients with at least one year follow-up. The 2 arms (56 control patients vs 53 aza maintenance patients) were comparable with regard to age (60-81 years, median 69 years), performance status, peripheral blood counts and cytogenetics. Although all patients had Results: A total of 52 patients received at least 1 cycle of aza. Subsequently, 44, 40, 34 and 32 patients received at least 3, 6, 9 and 12 cycles respectively. The maintenance treatment with aza was feasible, as, during the first 4 cycles, most treatments could be given according to schedule (87% Conclusion: Post-remission treatment with aza in older AML patients in CR/CRi after at least 2 cycles of intensive chemotherapy significantly improves DFS (p=0.005). When patients who received an allo HSCT were censored at time of transplant, the difference in OS between both arms was also significantly different (p=0.04), in favor of aza maintenance treatment Disclosures Huls: Celgene: Consultancy; JJ Gadeta (www.gadeta.nl): Consultancy, Equity Ownership, Patents & Royalties: on gdT cells and receptors and isolation strategies , Research Funding. Ossenkoppele: JJ Celgene: Honoraria, Research Funding; Roche: Honoraria; Karyopharm: Consultancy, Research Funding; Novartis: Research Funding.