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BACKGROUND: Several lifestyle factors have consistently been associated with survival of ovarian cancer, including body mass index, analgesic medications, physical activity, and smoking. Menopausal hormone therapy (HT), which can comprise of estrogen only (ET) or a combination of estrogen and progestin (EPT), has also been studied for its impact on ovarian survival. Randomized trials as well as a recent meta-analysis have shown better survival for women who took HT after their diagnosis in comparison to women who did not (HR=0.67, 95% CI 0.47-0.97). However, the relationship between use of HT prior to ovarian cancer diagnosis and survival is less clear. METHODS: Approximately 4,700 ovarian cancer patients across nine population-based studies participating in the Ovarian Cancer Association Consortium (OCAC) were included in our analysis. Information on HT use prior to diagnosis and potential confounders was obtained via phone or in-person interviews. To estimate the effect of pre-diagnostic HT use on ovarian cancer survival, hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated using Cox proportional hazards regression models with left and right truncation to account for different observation times. Effects by type and duration of HT use as well as disease histotype were also evaluated. RESULTS: Women who used HT before diagnosis had an 11% decreased risk of death in comparison to those who did not use HT (95% CI 0.82-0.97). Similar effects were observed for both types of HT (HR=0.86, 95% CI 0.75-0.98 for ET and HR=0.89, 95% CI 0.80-0.99 for EPT). In addition, there appeared to be a duration effect with longer HT use conferring better survival (p-trend=0.07 for ET, p-trend=0.003 for EPT). When histotype was considered, the overall decreased risk was driven by serous and mucinous ovarian cancers (HR=0.85, 95% CI 0.77-0.93 and HR=0.54, 95% CI 0.28-1.07, respectively). CONCLUSIONS: Use of HT after diagnosis also affords a survival advantage based on published observational and clinical trials data. We now report that pre-diagnosis HT use may confer a survival advantage among women with ovarian cancer. Interestingly, use of ET prior to diagnosis is associated with increased risk of ovarian cancer, but better survival among women diagnosed with the disease. These observations taken together demonstrate the complex nature of ovarian cancer. Citation Format: Alice W. Lee, Siri Peterson, Ashley Wiensch, Malcolm C. Pike, Celeste L. Pearce on behalf of the Ovarian Cancer Association Consortium. PRE-DIAGNOSIS USE OF MENOPAUSAL HORMONE THERAPY ASSOCIATED WITH BETTER OVARIAN CANCER SURVIVAL [abstract]. In: Proceedings of the 12th Biennial Ovarian Cancer Research Symposium; Sep 13-15, 2018; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2019;25(22 Suppl):Abstract nr DP-011.