Background: BM in patients (pts) with CRPC are associated with shorter overall survival (OS) and higher costs. BRI zoledronic acid and denosumab are frequently used to prevent skeletal-related events (SRE) in pts with CRPC and BM. AA is the most common 1st line therapy for men with metastatic CRPC. We aimed to assess the impact of BRI on OS and time to first SRE (ttSRE) of pts receiving 1st line treatment AA for CRPC with BM. Methods: A retrospective cohort of pts starting AA as 1st line therapy for CRPC with BM between 2013-2016 was identified through 8 hospitals’ IRB approved registries. Pts were classified by use of concomitant BRI and subgrouped by volume of disease (per E3805 definition) at AA start. Kaplan-Meier method and Cox models were used to assess OS and ttSRE with hazard ratio (HR) estimates (95% CI). Results: Of the 745 pts included (543 deaths), 529 (71.0%) had AA alone and 216 (29.0%) AA+BRI. Median follow-up was 23.5 months. Pts receiving concomitant BRI showed a significantly longer OS and a 35% reduced risk of death compared to AA alone (HR=0.65; 95% CI, 0.54-0.79; P<.0001). The OS benefit with BRI was greater for the subgroup with high volume disease (HV) (HR=0.51; 95% CI, 0.38-0.68; P<.0001). The cohort with AA+BRI had a significantly shorter ttSRE (HR=1.27; 95% CI; 1.0-1.60; P=.0439) and, notably, the risk of first SRE was more than doubled for the subgroup with LV (HR=2.29; 95% CI, 1.57-3.35; P<.0001). On MVA, BRI vs. no BRI, prior local therapy (PLT) vs. no PLT, LV vs. HV, baseline VAS pain ≤3 vs. >5, PS 0 vs. ≥1, and PSA are independently associated with longer OS. Conclusions: The addition of BRI to 1st line AA for CRPC men with BM was associated with improved OS, particularly in HV, and worsened ttSRE, more evident in LV. These data suggest a potentially different impact of concomitant BRI on HV vs. LV, which could affect clinical decision making.