Background: Blocking PD-1/PD-L1 pathways enhances anti-leukemia responses in murine AML (Zhang et al, Blood 2009). PD-1 positive CD8 T-cells are increased in bone marrow (BM) of pts with AML (Daver et al, ASH 2016). AZA up-regulates PD-1 in AML (Yang et al., Leukemia 2013). Methods: Pts were eligible if they had AML and failed prior therapy, had adequate performance status (ECOG ≤ 2), and organ function. AZA 75mg/m²Days 1-7 with nivolumab 3mg/kg on Day 1 and 14 was established as the recommended phase II dose. Courses were repeated every 4-5 weeks indefinitely. Responses were evaluated at the end of 3 courses. Results: 53 pts with med age 68 years (range, 44 – 90), secondary AML (43%), poor risk cytogenetics (43%), med prior regimens 2 (range, 1-7) have been enrolled. Common mutations included DNMT3A (n = 11), TP53 (n = 11), TET2 (N = 8), CEBPA (n = 8), ASXL1(n = 8). All 53 pts are evaluable for response: 11 (21%) achieved CR/CRi and 7 (14%) had hematologic improvement (HI) for an overall response rate of 35%. Additionally, 14 (26%) had ≥50% BM blast reduction, 3 (6%) had stable disease > 6 months, and 12 (23%) had progression. The CR/CRi have been durable with 9 of 11 (82%) pts with CR/CRi alive at 1 year, after censoring for SCT. Med survival for the 53 evaluable pts was 5.7 months (range, 0.9 – 16.2) and in the 27 salvage 1 pts was 9.3 months (range, 1.6 – 16.2). These compare favorably to historical survival with AZA-based salvage protocols at MDACC. Grade 3/4 and Grade 2 immune toxicities were observed in 7 (14%) and 6 (12%) pts, respectively. These responded rapidly to steroids and 12 of 13 pts were successfully rechallenged with nivolumab. Multicolor flow-cytometry data were available on pretherapy, end of cycle 1, and end of cycle 2 BM aspirates in 9 CR/CRi and 22 non-responders. Pts who achieved CR/CRi had higher pre-therapy total CD3 (P = 0.02) and higher CD8⁺ T-cells (P = 0.07) infiltrate in the BM. Responders demonstrated progressive increase in BM CD8⁺ and CD4⁺ infiltrate. Both responders and non-responders had increase in CTLA4⁺ CD8⁺ cells on therapy. Conclusions: Full dose AZA and nivolumab are tolerable and may produce durable responses in relapsed AML. Up-regulation of CTLA4 may be a mechanism of resistance to PD1 based therapies in AML. Clinical trial information: NCT02397720