Background-Atrial fibrillation (AF) is common among patients with end-stage renal disease, but few data are available on its prevalence among adults with chronic kidney disease (CKD) of lesser severity. Methods and Results-We evaluated the association of CKD with ECG-detected AF among 26 917 participants in the REasons for Geographic and Racial Differences in Stroke (REGARDS) study, a population-based cohort of African-American and white US adults >45 years of age. Estimated glomerular filtration rate (eGFR) was calculated using the abbreviated Modification of Diet in Renal Disease study equation and albuminuria was defined as a urinary albumin to creatinine ratio >30 mg/g. Participants were categorized by renal function: no CKD (eGFR >60 mL/min/1.73 m2 without albuminuria, n>21 081), stage 1 to 2 CKD (eGFR >60 mL/min/1.73 m2 with albuminuria n>2938), stage 3 CKD (eGFR 30 to 59 mL/min/1.73 m2, n>2683) and stage 4 to 5 CKD (eGFR =30 mL/min/1.73 m2, n>215). The prevalence of AF among participants without CKD, and with stage 1 to 2, stage 3, and stage 4 to 5 CKD was 1.0%, 2.8%, 2.7% and 4.2%, respectively. Compared with participants without CKD, the age-, race-, and sex-adjusted odds ratios for prevalent AF were 2.67 (95% confidence interval, 2.04 to 3.48), 1.68 (95% confidence interval, 1.26 to 2.24) and 3.52 (95% confidence interval, 1.73 to 7.15) among those with stage 1 to 2, stage 3, and stage 4 to 5 CKD. The association between CKD and prevalent AF remained statistically significant after further multivariable adjustment and was consistent across numerous subgroups. Conclusions-Regardless of severity, CKD is associated with an increased prevalence of AF among US adults. © 2011 American Heart Association, Inc.