Atrial fibrillation (AF) is a common arrhythmia that is a potent independent risk factor for stroke. The incidence of AF increase with age and most affected people have underlying cardiac disease. This study aimed to describe the prevalence of and risk factors for AF in Korean. In this study, 14,540 adults (male 6,573/female 7,967) > or =40 yr old received screening test for general health between April 2000 and December 2000. Participants answered questionnaires and underwent examinations that included blood pressure, electrocardiogram (ECG), total cholesterol, and fasting glucose. Data analysis was done by SPSS 10.0 for Windows. The prevalence of AF was 0.7% in people older than 40 yr and 2.1% in those older than 65 yr. The prevalence in men was 1.2% and women was 0.4% in people older than 40 yr. The prevalence in men was 3.3% and women was 1.1% in people older than 65 yr. Approximately 56.6% of individuals with AF are older than 65 yr. The prevalence of AF was higher at all age group in men than in women. Also, the prevalence of AF was highest in people older than 80 yr. In univariate analysis, male sex, old age (> or =65 yr), hypertension, diabetes mellitus, left ventricular hypertrophy in ECG, stroke, and cardiac disease were associated with an increased risk of AF. In multivariate analysis, however, risk factors of AF were male (odds ratio, OR 4.1; 95% confidence interval [CI] : 2.6 to 6.5; p=0.000), old age (OR 5.3; 95% CI:3.5 to 7.9; p=0.000), and cardiac disease (OR 19.8; 95% CI:12.3 to 31.8; p=0.000). In this study, the most potent risk factors of AF was cardiac disease.