Context: Obesity is associated with atrial enlargement and ventricular diastol ic dysfunction, both known predictors of atrial fibrillation(AF). However, it is unclear whether obesity is a risk factor for AF. Objective: To examine the asso ciation between body mass index(BMI) and the risk of developing AF. Design, Sett ing, and Participants: Prospective, communitybased observational cohort in Fra mingham, Mass. We studied 5282 participants(mean age, 57[SD, 13] years; 2898 wom en[55%]) without baseline AF(electrocardiographic AF or arterial flutter). Body mass index(calculated as weight in kilograms divided by square of height in met ers) was evaluated as both a continuous and a categorical variable(normal define d as < 25.0; overweight, 25.0 to < 30.0; and obese, ≥30.0). In addition to adju sting for clinical confounders by multivariable techniques, we also examined mod els including echocardiographic left atrial diameter to examine whether the infl uence of obesity was mediated by changes in left atrial dimensions. Main Outcome Measure: Association between BMI or BMI category and risk of developing newon set AF. Results: During a mean followup of 13.7 years, 526 participants(234 wo men) developed AF. Age adjusted incidence rates for AF increased across the 3 BM I categories in men(9.7, 10.7, and 14.3 per 1000 personyears) and women(5.1, 8 .6, and 9.9 per 1000 personyears). In multivariable models adjusted for cardio vascular risk factors and interim myocardial infarction or heart failure, a 4%i ncrease in AF risk per 1-unit increase in BMI was observed in men(95%confidenc e interval[CI], 1%-7%; P=.02) and in women(95%CI, 1%-7%; P=.009). Adjuste d hazard ratios for AF associated with obesity were 1.52(95%CI, 1.09-2.13; P=. 02) and 1.46(95%CI, 1.03-2.07; P=.03) for men and women, respectively, compare d with individuals with normal BMI. After adjustment for echocardiographic left atrial diameter in addition to clinical risk factors, BMI was no longer associat ed with AF risk(adjusted hazard ratios per 1-unit increase in BMI, 1.00[95%CI, 0.97-1.04], P=.84 in men; 0.99 [95%CI, 0.96-1.02], P=.56 in women). Conclusi ons: Obesity is an important, potentially modifiable risk factor for AF. The exc ess risk of AF associated with obesity appears to be mediated by left atrial dil atation. These prospective data raise the possibility that interventions to prom ote normal weight may reduce the population burden of AF.