Treatment with azacitidine (AZA), a demethylating agent, prolonged overall survival (OS) vs conventional care in patients with higher-risk myelodysplastic syndromes (MDS). As median survival with monotherapy is <2 years, novel agents are needed to improve outcomes. This phase 1b/2b trial (n=113) was designed to determine the maximum tolerated dose (MTD) or recommended phase 2 dose (RP2D) of panobinostat (PAN) + AZA (phase 1b) and evaluate early efficacy and safety of PAN+AZA vs AZA monotherapy (phase 2b) in patients with higher-risk MDS, chronic myelomonocytic leukemia, or oligoblastic acute myeloid leukemia with <30% blasts. The MTD was not reached; the RP2D was PAN 30 mg plus AZA 75 mg/m(2). More patients receiving PAN+AZA achieved a composite complete response ([CR] + morphologic CR with incomplete blood count + bone marrow CR (27.5% [95% CI, 14.6-43.9%]) vs AZA (14.3% [5.4-28.5%]). However, no significant difference was observed in the 1-year OS rate (PAN+AZA, 60% [50-80%]; AZA, 70% [50-80%]) or time to progression (PAN+AZA, 70% [40-90%]; AZA, 70% [40-80%]). More grade 3/4 adverse events (97.4 vs 81.0%) and on-treatment deaths (13.2 vs 4.8%) occurred with PAN+AZA. Further dose or schedule optimization may improve the risk/benefit profile of this regimen.Leukemia accepted article preview online, 26 May 2017. doi:10.1038/leu.2017.159.