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Underuse of oral anticoagulation in stroke prevention in atrial fibrillation is common; however, it remains unknown how it varies geographically. The objective of this study was to evaluate geographic variation in oral anticoagulation use and in the initiation of new oral anticoagulants (NOACs).Using 2013 to 2014 claims data from a 5% random sample of Medicare beneficiaries, we identified patients newly diagnosed with atrial fibrillation who initiated NOACs (n=8659), warfarin (n=11 771) or no oral anticoagulation therapy (n=18 226) in 2013 to 2014. Each patient was assigned to 1 of the 306 Dartmouth hospital-referral regions based on his/her zip code. We constructed logistic regressions to calculate the mean adjusted probability of initiating oral anticoagulation and the mean adjusted probability of initiating an NOAC among patients on oral anticoagulation in each hospital-referral region, after adjustment for demographic, clinical, and provider characteristics and type of insurance. Finally, we computed the correlation between 2 probabilities.Higher in the Midwest (0.54) and Northeast (0.54) and lowest in the South (0.47), the mean adjusted probability of initiating oral anticoagulation was 0.51, ranging from 0.32 to 0.72. The mean adjusted probability of being prescribed an NOAC among those on oral anticoagulation was 0.42 and was highest in the South (0.50) and lowest in the Midwest (0.36) and Northeast (0.39). In areas with the lower use of oral anticoagulation, patients on any oral anticoagulation therapy had a higher likelihood of being prescribed an NOAC (correlation coefficient, −0.16; P=0.006).Large geographic variation exists in oral anticoagulation use in atrial fibrillation. The use of oral anticoagulation is lower in the South, where the rates of stroke are unusually high. In the future, it will be important to analyze whether the high rates of stroke in the South can be partially attributed to the underuse of oral anticoagulation in this region.