New-onset atrial fibrillation (AF) occurs frequently in association with myocardial infarction (MI), with an incidence ranging from 5% to 23%. Several studies demonstrated an independent risk of AF for mortality. Conversely, others did not show a significant independent effect, thus suggesting that the simultaneous onset of AF and MI merely reflects the severity of the underlying disease or comorbidities, or both. The concomitant presence of AF and MI is associated with a higher re-infarction rate, a higher stroke rate, and a higher risk for heart failure, and may also increase the risk for sudden cardiac death. Interestingly, even transient, self-terminating AF during ST-elevation myocardial infarction (STEMI) was shown to be related to a significantly higher stroke rate in long-term follow-up.