We report a clinical trial testing vaccination of autologous myeloblasts admixed with granulocyte-macrophage colony-stimulating factor secreting K562 cells after allogeneic hematopoietic stem cell transplantation (HSCT). Patients with myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML) with ≥5% marrow blasts underwent myeloblast collection before HSCT. At approximately day +30, 6 vaccines composed of irradiated autologous myeloblasts mixed with GM-K562 were administered. Tacrolimus-based graft-versus-host disease (GVHD) prophylaxis was not tapered until vaccine completion (∼day 100). Thirty-three patients with AML (25) and MDS (8) enrolled, 16 (48%) had ≥5% marrow blasts at transplantation. The most common vaccine toxicity was injection site reactions. One patient developed severe eosinophilia and died of eosinophilic myocarditis. With a median follow-up of 67 months, cumulative incidence of grade 2-4 acute and chronic GVHD were 24% and 33%, respectively. Relapse and nonrelapse mortality were 48% and 9%, respectively. Progression-free survival (PFS) and overall survival (OS) at 5 years were 39% and 39%. Vaccinated patients who were transplanted with active disease (≥5% marrow blasts) had similar OS and PFS at 5 years compared with vaccinated patients transplanted with <5% marrow blasts (OS, 44% vs 35%, respectively, P = .81; PFS, 44% vs 35%, respectively, P = .34). Postvaccination antibody responses to angiopoietin-2 was associated with superior OS (hazard ratio [HR], 0.43; P = .031) and PFS (HR, 0.5; P = .036). Patients transplanted with active disease had more frequent angiopoeitin-2 antibody responses (62.5% vs 20%, P = .029) than those transplanted in remission. GM-K562/leukemia cell vaccination induces biologic activity, even in patients transplanted with active MDS/AML. This study is registered at www.clinicaltrials.gov as #NCT 00809250.