In a study published in the journal circulation the official journal of the American Heart Association the relation between the onset of atrial fibrillation at the time of a myocardial infarction (heart attack) was assessed. Atrial fibrillation (AF) often coexists with myocardial infarction (MI), yet its prognostic influence is disputed. In a community-based cohort of 3220 patients hospitalized with incident (first-ever) MI from 1983 to 2007 in Olmsted County, MN. Atrial fibrillation was identified by diagnostic codes and ECG. Outcomes were all-cause and cardiovascular death. Atrial fibrillation before MI was identified in 304 patients, and 729 developed AF after MI (218 [30%] within 2 days, 119 [16%] between 3 and 30 days, and 392 [54%] >30 days post-MI). The cumulative incidence of AF after MI at 5 years was 19% and did not change over the calendar year of MI (the incidence of AF was the same regardless of when the MI occurred). During a mean follow-up of 6.6 years, 1638 deaths occurred. AF was associated with an increased risk of death independent of clinical characteristics at the time of MI and heart failure. This risk differed markedly according to the timing of AF, and was the greatest for AF occurring >30 days post MI for AF within 2 days, for AF between 3 and 30 days, and 2.58 for AF >30 days post MI. In the community, AF is frequent in the setting of MI. Atrial fibrillation carries an excess risk of death, which is the highest for AF developing >30 days after MI.