Background: Bone modifying agents (BMAs) do not prevent skeletal related events (SREs) among patients with metastatic, castration-sensitive prostate cancer (mCSPC). Within this population, BMAs are recommended for prevention of osteoporotic fractures but not for SRE prevention. Utilization patterns of BMAs among patients with mCSPC have not been described, and it is unknown whether these patients may receive BMAs intended for SRE prevention unnecessarily. We conducted this study to measure BMA use among mCSPC patients who are unlikely to have an indication for osteoporotic fracture prevention. Methods: We used linked SEER-Medicare data. Our cohort included men newly diagnosed with de-novo stage IV prostate adenocarcinoma during 2007-2015. We included those age > = 66 at diagnosis, who lived at least 60 days after diagnosis, had continuous enrollment in Medicare Parts A and B from 180 days prior to diagnosis through the outcome period and Part D from diagnosis through outcome period, and who received androgen deprivation or antiandrogen therapy after diagnosis. We excluded patients who had previously received BMAs, or who had existing osteoporosis, osteopenia, hypercalcemia, or prior bone fracture. Our primary outcome was receipt of a BMA (zoledronic acid or denosumab) within 180 days of prostate cancer diagnosis, during which time the development of castrate-resistant disease is unlikely to arise. We also assessed receipt of BMAs within 90 days of diagnosis as a secondary outcome. We used multivariable logistic regression to assess patient factors associated with BMA use. Results: Our sample included 2,998 patients. Median age at diagnosis was 77. 77% of patients were white, 12% were Black, and 11% were of other race/ethnicity. 77% of patients were treated in the physician office setting and 23% the hospital outpatient setting. Overall, 24% received a BMA within 180 days of diagnosis; 18% did within 90 days. BMA therapy was more common among patients treated in the hospital outpatient vs. physician office setting (OR 1.25, 95%CI 1.03-1.52). Age, race/ethnicity, comorbidity, and renal disease were not associated with likelihood of BMA therapy. There was substantial variation across the study period: 19% of patients received BMAs from 2007-2009 (19% zoledronic acid, 0% denosumab), and 29% from 2012-2015 (9% zoledronic acid, 21% denosumab). Conclusions: Use of BMAs among patients with mCSPC is common. In this cohort of mCSPC patients, who were unlikely to have a BMA indication for osteoporotic fracture prevention based on the absence of history of osteoporosis, osteopenia, or fracture, nearly one-quarter received BMAs. The increase in BMA use which occurred during the 2007-2015 study period appears to have been driven by increased utilization of denosumab following its approval in 2010. Use of BMAs in patients with mCSPC without a need for osteoporotic fracture prevention may constitute overuse.