Measurable residual disease (MRD) can predict relapse in patients with advanced myelodysplastic neoplasms (MDS) or acute myeloid leukemia (AML). We report the long-term efficacy and safety of MRD-guided preemptive azacitidine treatment to prevent relapse in the phase 2 Relapse Prevention With Azacitidine (RELAZA2) trial. Patients with MDS or AML after either intensive chemotherapy only or consecutive allogeneic stem cell transplantation were prospectively screened for imminent relapse by molecular MRD assessment. Patients who became MRD positive (MRDpos) during screening received azacitidine for up to 2 years to prevent relapse. The primary end point was the proportion of patients alive and relapse-free 6 months after azacitidine start. Of 357 patients screened, 119 (33.3%) became MRDpos, of whom 95 (79.8%) were eligible for azacitidine treatment. The primary end point was met; 60 (63%) patients were relapse free (95% confidence interval, 54-71; P< .0001) 6 months after azacitidine initiation with no new safety signals. Of 60 patients achieving MRD response during the first 6 cycles of azacitidine, 31 (52%) maintained response without hematological relapse for ≥2 years after azacitidine initiation. The median treatment-free duration after azacitidine discontinuation was 20.8 months; the longest ongoing response was 104 months. After a median follow-up of 6.6 years, 15 initial responders (25%) remained alive and in remission. Among screened patients who remained continuously MRD negative, 60-month overall survival and relapse-free survival were 88% and 79%, respectively. Patients with continuously negative MRD display a very favorable prognosis. Most patients with MRD positivity can be effectively treated with azacitidine with potential long-term remission even after termination of azacitidine. This trial was registered at www.clinicaltrials.gov as #NCT01462578.