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To study the association between time in therapeutic range (TTR) during warfarin therapy and risk of dementia in a population-based cohort of incident atrial fibrillation (AF).We conducted an observational population-based study of 2800 nondemented patients with incident AF from January 1, 2000, through December 31, 2010. The association of incident dementia with warfarin therapy and TTR was examined using Cox proportional hazards regression models.Mean patient age was 71.2 years; 53% were men (n=1495), and warfarin was prescribed to 50.5% (n=1414) within 90 days of AF diagnosis. Incident dementia diagnosis occurred in 357 patients (12.8%) over a mean ± SD follow-up of 5.0±3.7 years. After adjusting for confounders, warfarin therapy was associated with a reduced incidence of dementia (hazard ratio [HR], 0.80; 95% CI, 0.64-0.99). However, only those in the 2 highest quartiles of TTR were associated with lower risk of dementia. A 10% increase in TTR with a 10% reduction in time spent in the subtherapeutic (HR, 0.71; 95% CI, 0.64-0.79) and supratherapeutic (HR, 0.67; 95% CI, 0.57-0.79) ranges were associated with decreased risk of dementia.In the community, warfarin therapy for AF is associated with a 20% reduction in risk of dementia. Increasing TTR on warfarin is associated with reduced risk of dementia. The risk of dementia was reduced with a reduction in time spent in subtherapeutic and supratherapeutic international normalized ratio range. Effective anticoagulation may prevent cognitive impairment in patients with AF.