This study examined the prognostic significance of atrial fibrillation(AF) in a community-based cohort. AF,the most common cardiac dysrhythmia, frequently oc curs in the presence of concomitant medical illness. Population-based studies h ave associated AF with excess mortality, and this risk of death is independent o f concomitant cardiovascular disease. The effect of noncardiovascular medical il lnesses on mortality in patients who have AF has not been determined. We examine d a community-based cohort of 390 residents of Olmsted County, Minnesota, who h ad newly diagnosed AF; patients were identified retrospectively but followed pro spectively. The cohort in cluded all patients who had electrocardiographically proved first-onset AF; p atients were not excluded on the basis of medical illness. Mean age of the cohor t was 73±14 years(56%were men). Mean follow-up was 2.7±1.7 years. Onset of A F frequently occurred during hospitalization(78%). One hundred sixty-six death s occurred, a death rate significantly higher than expected for the cohort. Most deaths had a noncardiovascular cause. This trend was maintained for patients wh o had no previous cardiovascular disease and for those who had a cardiovascular diagnosis at the time AF was diagnosed. AF is observed frequently among hospital ized patients who are medically ill. The survival rate of these patients is low, but AF may be only a minor component of the excess mortality. This study examined the prognostic significance of atrial fibrillation (AF) in a community-based cohort. AF, the most common cardiac dysrhythmia, frequently oc curs in the presence of concomitant medical illness. Population-based studies h ave associated AF with excess mortality, and this risk of death is independent of concomitant cardiovascular disease. The effect of noncardiovascular medical il lnesses on mortality in patients who have AF has not been determined. We examine da community-based cohort of 390 residents of Olmsted County, Minnesota, who h ad newly diagnosed AF; patients were identified retrospectively but followed pro spectively. The cohort in cluded all patients who had electrocardiographically verified first-onset AF; p atients were not excluded on the basis of medical illness. Mean age of the cohor t was 73 ± 14 Mean follow-up was 2.7 ± 1.7 years. Onset of AF occurred during hospitalization (78%). One hundred sixty-six death s occurred, ad This trend was maintained for patients wh o had no previous cardiovascular disease and for those who who had a cardiovascular diagnosis at the time AF was diagnosed. AF is observed as active hospital ized patients who are medically ill. The survival rate of these patients is low, but AF may be only a minor component of the excess mortality.