Abstract Epithelial ovarian cancer (EOC) is the second most common gynecologic malignancy in the United States, but the leading cause of gynecologic cancer death. Despite many women achieving remission with first-line therapy, which includes cytoreductive surgery and platinum-taxane chemotherapy, up to 80% of patients will recur and require additional treatment. The interval from last platinum chemotherapy to recurrence has important prognostic and therapeutic implications for patients. Patients with platinum-resistant EOC have fewer treatment options, worse prognosis, and their disease is seldom curable. Antibiotic therapy is frequently used during cancer treatments in patients with EOC for both prophylaxis and treatment of infections after surgery or through chemotherapy. While critical for the care of the patient, we now appreciate that the microbiome has many roles in maintenance of human health and is increasingly linked with many disease states including obesity, cardiovascular disease, and cancer. Recent evidence suggests the gut microbiome may modulate responses to cancer treatment, including traditional chemotherapy and immunotherapy. Given that antibiotics may lead to microbiome disruption for care of EOC patients and lack of understanding of the impact of these antibiotics on tumor progression, in preclinical models, we assessed the impact of microbiome disruption on EOC progression and cisplatin sensitivity. We established a necessary role for the microbiome in suppression of tumor growth and cisplatin response in two mouse models of EOC. Following treatment with antibiotics (ampicillin, neomycin, vancomycin, and metronidazole), murine ID8 or ID8-VEGF EOC that were injected intraperitoneally into C57Bl/6J mice exhibited accelerated tumor growth compared to non-antibiotic treated controls. Tumor growth was monitored by ultrasound weekly. ID8 and ID8-VEGF tumors in antibiotic-treated mice exhibited reduced sensitivity to cisplatin compared to non-antibiotic controls. Mice treated with antibiotics had significantly worse survival compared to non-antibiotic controls. We confirmed depletion of the gut microbiome based on dilated cecum and reduced microbial 16S rRNA concentration in stool of antibiotic-treated compared to control mice. The accelerated tumor growth and cisplatin resistance was not dependent on an intact immune system as we replicated the effect in immune-deficient mice. In mechanistic studies we determined that disruption of the microbiome led to increased cancer stem cells that was further augmented by cisplatin treatment. Collectively, these studies indicate an intact microbiome provides a tumor-suppressive microenvironment and enhances sensitivity to cisplatin. Future studies will assess whether individual microbial communities are sufficient to reverse the accelerated tumor growth and resistance to cisplatin observed in antibiotic-treated mice and use of selective antibiotics to target pathogenic bacteria while sparing beneficial microbes. Citation Format: Laura Chambers, Emily Esakov, Chad Braley, Leila Edelman, Roberto Vargas, Justin Lathia, Chad Michener, Ofer Reizes. Gut microbiome attenuates epithelial ovarian cancer growth and sensitivity to cisplatin: New opportunities for ovarian cancer treatments [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research; 2019 Sep 13-16, 2019; Atlanta, GA. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(13_Suppl):Abstract nr B69.