Background: Frontline AML induction produces 60-70% complete remission (CR) rates but relapse is a major source of failure. Beside stem cell transplant (SCT), few options exist for post-CR maint in HR pts. Prior maint studies of cytotoxics were unsuccessful. Post SCT immune surveillance via tumor-specific cytotoxic T-cells may be important in suppressing AML relapse. Immune checkpoint inhibitors may restore host immune surveillance in post-CR maint. Methods: This is a pilot phase II study of nivo maint in HR AML pts in CR, ineligible for SCT. Pts ≥ 18 yrs with a HR feature in 1st CR (CR1) or any pt in 2^nd CR (CR2) who received induction &≥ 1 consolidation cycle & were within 12 months (mos) of CR were eligible. Treatment was: nivo 3mg/kg IV Q2 weeks for 6 mos, then Q4 weeks until 12 mos on study, & then Q3 mos until relapse. All pts had baseline cytogenetic (CG) & molecular testing, & minimal residual disease (MRD) assessment by flow cytometry. Blood & marrow samples were collected for immune correlatives. CR duration (CRd) is compared to a similar historical cohort with median (med) of 8 mos. Results: 14 pts (med age 56 yrs): 11 pts were in CR, CRp (1), & CRi (2) at enrollment; 11 pts (79%) were in CR1, 2 pts (14%) in CR2, & 1 pt (7%) in CR4 was treated. Baseline mutations: TP53 (n = 3), DNMT3a (2), IDH2 (2), NPM1 (2), TET2 (3). HR features: 5 (36%) persistent MRD, 4 (29%) adverse CG, 1 (7%) adverse mutation alone, 1 t-AML (7%) & 3 pts (21%) in ≥ CR2. Pts received a med of 4 (1-17) cycles of therapy. At med F/U of 11 mos (1.4 – 26), med CRd was not reached. 6- & 12-mo rates of CRd were 79% & 71%, respectively. The 12- & 18-mo estimated OS were 86% & 67%, respectively. Therapy was well tolerated; 5 pts had grade 3/4 immune-related events. 1 pt had thyroiditis, treated with steroids & hormone replacement; 1 pt with transaminitis responded to dose interruption; 2 pts had pneumonitis treated with steroids & dose interruption. All 4 pts resumed rx after interruption. 1 pt had autoimmune hemolytic anemia & came off study. Most pts have detectable MRD while on therapy & in CR; 1 pt cleared MRD & 1 pt normalized CG. Conclusions: Maint nivo is safe & feasible in HR AML. The study continues to surpass expected rate of 6-mo CRd of HR pts. Correlatives profiling the immune repertoire are being analyzed. Clinical trial information: NCT02532231