Abstract Most patients diagnosed with pancreatic adenocarcinoma (PDAC) survive less than 5 years, but a very small subset of patients survive longer. The factors that determine the long-term survivorship remain elusive. Recently, studies have shown that bacteria can be found in PDAC which may influence therapy responses. In this study, we aimed to determine if the tumor microbiome, and its associated immune responses, can guide long-term survivorship in resected PDAC patients. Using 16S rRNA gene sequencing, we analyzed the tumor microbiome composition and immunoprofile in PDAC patients who survived less than 5 years (short term survivors, STS) versus those who survived more than 5 years (long term survivors, LTS) in two independent cohorts of patients from two institutions (MD Anderson Cancer Center and Johns Hopkins University). We found higher alpha-diversity in the tumor microbiome from LTS compared to STS PDAC patients. Additionally, we found greater densities of immune cells in the LTS compared to STS, with significant correlation with alpha-diversity. Taken together, our study demonstrates that the PDAC microbiome composition may influence the host immune response and the natural history of the disease. E.R. and Y.Z. contributed equally to this work. Citation Format: Erick M. Riquelme, Yu Zhang, Liangliang Zhang, Montiel Maria, Zoltan Michelle, Wenli Dong, Pompeyo Quesada, Ismet Sahin, Vidhi Chandra, Anthony San Lucas, Paul Scheet, Hanwen Xu, Samir M. Hanash, Lei Feng, Nadim Ajami, Joseph Petrosino, Christine B. Peterson, Deborah Nejman, Michael P. Kim, Cynthia L. Sears, Laura D. Wood, Anirban Maitra, Ravid Straussman, Matthew Katz, James Robert White, Robert Jenq, Jennifer Wargo, Florencia McAllister. Pancreatic tumor microbiome and associated immune responses determine clinical outcomes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2829.