We prospectively examined the association between body mass index and risk of atrial fibrillation or flutter among 47,589 participants (mean age 56 years) in the Danish Diet, Cancer and Health Study. During follow-up (mean 5.7 years), atrial fibrillation or flutter developed in 553 subjects (372 men and 181 women). We found that the risk of atrial fibrillation increased substantially as body mass index increased in both men and women.