BACKGROUND AND PURPOSE-: Warfarin reduces stroke risk by approximately 60% in patients with atrial fibrillation (AF). Differences in awareness and treatment of AF may contribute to racial and geographic disparities in stroke mortality. The objective was to examine predictors of awareness of the diagnosis of AF and treatment with warfarin. METHODS-: REasons for Geographic and Racial Differences in Stroke (REGARDS) is a national, population-based, longitudinal study of 30 239 blacks and whites ≥45 years old with oversampling from blacks and the southeastern stroke belt states. Participants were enrolled January 2003 to October 2007. Data were collected using telephone interview, in-home evaluation, and self-administered questionnaires. The main variable of awareness of AF was defined by a positive answer to "Has a doctor or other health professional ever told you that you had atrial fibrillation?" and whether there was evidence of treatment on the basis of an in-home medications inventory. RESULTS-: From baseline electrocardiograms, 432 individuals (88 black and 344 white) had AF. Of these, 88% (360 of 409) had at least 1 additional CHADS2stroke risk factor and 60% (258 of 432) were aware of their AF. The odds of blacks being aware of their AF were one third that of whites (OR=0.32; 95% CI: 0.20 to 0.52). Among those aware, the odds of blacks being treated with warfarin were only one fourth as great as whites (OR=0.28; 0.13 to 0.60). CONCLUSION-: Blacks were less likely than whites to be aware of having AF or to be treated with warfarin. Potential reasons for the racial disparity in warfarin treatment warrant further investigation. © 2010 American Heart Association, Inc.