Almost all studies show that atrial fibrillation (AF) is associated with increased mortality. What is less certain is whether this association is a straightforward cause-and-effect relationship, or if AF is merely a marker of severity of cardiovascular disease(s) or the aging process. AF can lead to the worsening of left ventricular filling, contribute to loss of atrioventricular synchrony, affect cardiac remodeling, and even cause a tachycardia-induced cardiomyopathy. AF could be a marker for underlying atherosclerotic disease that itself determines mortality, or the increased oxygen consumption associated with an increasing ventricular rate may lead to ischemia secondary to increased myocardial consumption and precipitate acute coronary syndromes. Although it is generally accepted that the stasis of atrial blood in AF promotes clot formation, studies have shown increases in specific coagulation factors—all of which have the ability to increase morbidity and/or mortality through their elevations. Another possibility is that AF is not the cause of the hypercoagulable state, but is instead a marker of such a state.