BACKGROUND:The frequency of cardiac rhythm abnormalities and their risk factors in community-dwelling adults are not well characterized. METHODS:We determined the frequency of rhythm abnormalities in the UK Biobank, a national prospective cohort. We tested associations between risk factors and incident rhythm abnormalities using multivariable proportional hazards regression. RESULTS:Of 502 627 adults (median age, 58 years interquartile range, 13; 54.4% women), 2.35% had a baseline rhythm abnormality. The prevalence increased with age with 4.84% of individuals aged 65 to 73 years affected. During 3 368 332 person-years of follow-up, 15 906 new rhythm abnormalities were detected (4.72 per 1000 person-years; 95% confidence interval CI4.65–4.80). Atrial fibrillation (3.11 per 1000 person-years; 95% CI3.05–3.17), bradyarrhythmias (0.89 per 1000 person-years; 95% CI0.86–0.92), and conduction system diseases (1.06 per 1000 person-years; 95% CI1.02–1.09) were more common than supraventricular (0.51 per 1000 person-years; 95% CI0.48–0.53) and ventricular arrhythmias (0.57 per 1000 person-years; 95% CI0.55–0.60). Older age (hazard ratio HR2.35 per 10-year increase; 95% CI2.29–2.41; P<0.01), male sex (HR1.83; 95% CI1.76–1.89; P<0.01), hypertension (HR1.49; 95% CI1.44–1.54; P<0.01), chronic kidney disease (HR1.95; 95% CI1.67–2.27; P<0.01), and heart failure (HR1.99; 95% CI1.76–2.26; P<0.01) were associated with new rhythm abnormalities. CONCLUSIONS:The frequency of rhythm abnormalities in middle-aged to older community-dwelling adults is substantial. Atrial fibrillation, bradyarrhythmias, and conduction system diseases account for most rhythm conditions.