Objective: To characterize the rate of guideline-concordant initiation of oral antico-agulation (OAC) among elderly Veterans with atrial fibrillation (AF) and high stroke risk. Data Sources/Study Setting: Veterans Health Administration (VHA) Corporate Data Warehouse (CDW) linked with Medicare claims 2011-2015. Study Design: We identified 6,619 elderly, high stroke-risk patients with a new episode of AF initially diagnosed in the VHA during fiscal years 2012-2015. We used logistic regression to estimate marginal effects of associations between patient char-acteristics and OAC initiation within 90 days of the first AF episode. Data Extraction Methods: We identified OACs using generic drug names. We calculated comorbidities and risk scores using diagnosis codes from 1 year of baseline data. Principal Findings: Overall, 66.5% of Medicare-eligible Veterans with AF at high risk of stroke initiated an OAC within 90 days. We found lower initiation rates for patients enrolled in Medicare Part D and those ineligible for drug co-payment subsidies. OAC initiation rates increased during the study among VHA-reliant patients but not among dual VHA-Part D enrollees. Conclusions: One-third of elderly Veterans at risk of stroke are not receiving recommended therapy. Increased coordination between Medicare and VHA providers may lead to improvements in anticoagulation quality and stroke prevention.