Background: The American College of Cardiology, American Heart Association, and European Society of Cardiology Board (ACC/AHA/ESC) 2001 guidelines for management of patients with atrial fibrillation (AF) include a new classification system consisting of 4 categories: first-detected episode; recurrent paroxysmal (self-terminating); recurrent persistent (requiring cardioversion); and permanent. The frequency of hospital discharges within these categories has not been reported. Methods: The new classification system was applied to 135 consecutive hospital discharges with a principal diagnosis of AF. Results: Classification of AF in these discharged patients included 74 (55%) with first-detected episode; 28 (21%) with recurrent paroxysmal AF; 17 (13%) with recurrent persistent AF; and 16 (12%) with permanent AF. Hypertension (n=48; 35%) was the most common primary cause of AF, followed by alcohol related (n=23; 17%), coronary artery disease (n=20; 15%), and valvular heart disease (n=17; 12%). For the 102 patients with first-detected and recurrent paroxysmal AF, 71 (69%) converted spontaneously to normal sinus rhythm within 48 hours of admission. Of the 48 patients with a discharge diagnosis of AF, 32 (67%) were receiving anticoagulation therapy. Conclusions: Most hospital discharges with a principal diagnosis of AF represent the first-detected episode. Diverse causes contribute to AF, and to examine them would help direct therapy. Importantly, in our analysis, 69% of those patients with first-detected or recurrent paroxysmal AF converted spontaneously to normal sinus rhythm within 48 hours of admission.