Background: GOG-0218, a double-blind placebo (PLA) controlled phase III trial of 1873 pts with advanced EOC, compared CP with either PLA (CPP), BEV 15 mg/kg q3w → PLA (CPB15), or BEV 15 mg/kg q3w → BEV for 15 mo (CPB15+). Study results demonstrated significantly improved progression-free survival (PFS), but no overall survival (OS) benefit with BEV. Blood samples were collected for BM analyses. Methods: Plasma samples were analyzed via multiplex ELISA technology for 5 pre-specified candidate BMs (IL6, Ang-2, osteopontin (OPN), stromal cell-derived factor-1 (SDF-1), and VEGF-D). Associations of BMs’ baseline values with PFS and OS were assessed as continuous variables using multivariate Cox proportional hazard regression models including the covariates age, stage, debulking status and performance status. Predictive markers were identified using treatment by marker interaction term in multivariate Cox models. Results: Baseline samples were available from 752 pts. According to our pre-specified analysis plan, IL6 was strongly predictive of a therapeutic advantage with BEV for both PFS and OS. Pts with high IL6 levels treated with BEV had longer PFS (14.2 vs. 8.7 mo) and OS (39.6 vs. 33.3 mo) compared to PLA. Increasing IL6 levels were associated with greater benefit from BEV. For PFS: Low IL6, HR 0.87 [CI 0.70-1.08]; Median, HR 0.77 [CI 0.58–1.01]; High, HR 0.67 [CI 0.48–0.94]; interaction p=0.009; for OS: Low IL6, HR 1.07 [CI 0.84–1.37]; Median, HR 0.92 [CI 0.67–1.26]; and High, HR 0.79 [CI 0.54–1.16]; interaction p=0.005. Additionally, both IL6 and OPN were found to be negative prognostic markers for both PFS and OS (p<0.001). No predictive associations were seen for other BMs tested. Conclusions: The inflammatory cytokine IL6 is highly predictive of therapeutic benefit from BEV when combined with CP. Plasma IL6 may define those EOC pts more or less likely to benefit from the addition of BEV to standard chemotherapy. These data are consistent with previous findings in which IL6 predicted benefit from BEV and pazopanib in renal cell cancer, highlighting the importance of inflammation in ovarian and renal cancers.