Background A community-based study was conducted to estimate the prevalence of atrial fibrillation (AF) in Japanese adults aged ≥40 years in Kurashiki-city and to examine associated risk factors. Methods and Results Adult residents (≥40-year-old) were examined in the Kurashiki-city Annual Medical Survey (KAMS) provided by the Kurashiki-city Public Health Center from May to December 2006. KAMS consisted of medical interviews, physical examination, blood testing and electrocardiography to assess the presence of AF. All participants were divided into AF and non-AF groups, their clinical characteristics compared, and the risk factors associated with AF investigated. Of 246,246 adult residents, 41,436 (age: 72.1±11.3 years; 13,963 men) underwent the KAMS. Estimated overall prevalence of AF was 1.6%. Multivariate analysis demonstrated that AF was independently associated with cardiac disease (odds ratio (OR), 9.00; 95% confidence interval (CI), 7.65-10.6; p<0.001), chronic kidney disease (OR, 1.76; 95%CI, 1.49-2.07; p<0.001), male sex (OR, 1.59; 95%CI, 1.34-1.85; p<0.001), and diabetes mellitus (OR, 1.46; 95%CI, 1.20-1.78; p<0.001). Hypercholesterolemia was highly associated with the absence of AF (OR, 0.54; 95%CI, 0.45-0.64, p<0.001). Conclusions Prevalence of AF in Japan is approximately 1.6%, lower than that in Western populations. Male sex, and cardiac and chronic kidney diseases are associated with the presence of AF. (Circ J 2008; 72: 909 - 913)