BACKGROUND: Prior studies have shown a treatment gap in oral anticoagulation (OAC) use among patients with atrial fibrillation yet have incompletely characterized factors associated with failure to treat and subsequent outcomes in contemporary practice. METHODS: Using data collected between June 2010 and August 2011 from 174 ambulatory care sites in the Outcomes Registry for Better Informed Treatment of Atrial Fibrillation, we identified factors associated with absence of OAC via stratified logistic regression. Using weighted Cox regression, we assessed the association between OAC non-use and subsequent outcomes over 2.5 years. RESULTS: Among 9553 patients, 2202 (23.0%) were not on OAC. Among OAC nonrecipients, 1846 (83.8%) had a CHA(2)DS(2)-VASc score >= 2. Factors independently associated with OAC non-use included atrial fibrillation type (paroxysmal odds ratio [OR] 0.73, 95% confidence interval [CI] 0.54-0.99; persistent OR 0.14, 95% CI 0.10-0.21; permanent OR 0.35, 95% CI 0.25-0.49; reference = new-onset), left atrial diameter enlargement (mild OR 0.80, 95% CI 0.66-0.97; moderate 0.58, 95% CI 0.47-0.73; severe 0.53, 95% CI 0.42-0.68; reference = normal diameter), and age >80 years (OR 1.04, 95% CI 1.02-1.08). Untreated patients had a higher risk of death (adjusted hazard ratio [HR] 1.22, 95% CI 1.05-1.41), a lower bleeding risk (adjusted HR 0.35, 95% CI 0.15-0.81), and a nonsignificant trend toward higher risk of stroke/non-central nervous system embolism/transient ischemic attack than those treated (adjusted HR 1.18, 95% CI 0.91-1.54). CONCLUSIONS: A majority of atrial fibrillation patients not treated with an OAC in current community practice meet guideline indications for treatment. Atrial fibrillation burden, chronicity, and comorbidity are associated with nontreatment. Untreated patients are at increased risk for adverse outcomes. Published by Elsevier Inc.