Background: Abiraterone and enzalutamide are oral medications approved by the Food & Drug Administration in 2011 and 2012 to treat men with advanced castration-resistant prostate cancer. Most men with advanced prostate cancer are over age 65 and thus eligible for Medicare Part D. We conducted a study to better understand the early dissemination of these drugs across the United States using national Medicare Part D data. Methods: We evaluated the number of prescriptions for abiraterone and enzalutamide by provider specialty and hospital referral region (HRR) using Medicare Part D and Dartmouth Atlas data. We categorized HRRs by abiraterone and enzalutamide prescriptions, adjusted for prostate cancer incidence, and examined factors associated with regional variation using multilevel regression models. Results: Among all providers who wrote prescriptions for abiraterone or enzalutamide in 2013 (n=2121), 87.5% were medical oncologists, 3.3% urologists, and 9.2% were listed as other provider specialties. Among those who prescribed either drug, 5% of providers were responsible for 75% of the claims for abiraterone, and 7% were responsible for 75% of the claims for enzalutamide. Some HRRs demonstrated low-prescribing rates despite average medical oncology and urology physician workforce density. Conclusions: The majority of prescriptions written for abiraterone and enzalutamide through Medicare Part D in 2013 were written by a minority of providers with marked regional variation across the United States. Better understanding the early national dissemination of these effective but expensive drugs can help inform strategies to optimize introduction of new, evidence-based advanced prostate cancer treatments.