Study Objective: We sought to describe the feasibility of an emergency department observation unit (EDOU) treatment protocol for the management of uncomplicated acute-onset atrial fibrillation (AAF). Methods: This descriptive case series took place at a major suburban, university-affiliated teaching hospital. Patients were prospectively enrolled in an EDOU treatment protocol if they had uncomplicated AAF that failed initial ED attempts to convert to sinus rhythm. In the EDOU, patients underwent ECG monitoring, serial creatine kianse MB measurements, and further rate control with optional electrical cardioversion. Primary outcomes measured were EDOU rate of conversion to sinus rhythm, rate of discharge home, length of stay, positive diagnostic outcomes, complications of AAF, and 7-day return visits. Results: Sixty-seven patients were studied. Patients were symptomatic for a median of 4.0 hours, had mean initial ED pulse rates of 137±23 beats/min, and spent 4.7±2.2 hours in the ED before transfer to the EDOU. While in the EDOU, 55 (82%) patients converted to sinus rhythm. Five (7%) patients were admitted because of positive test results: 2 for myocardial infarction, 2 for fever, and 1 for ventricular tachycardia. Twelve (18%) patients remained in atrial fibrillation, with 9 admitted and 3 discharged. Overall, 81% of patients were discharged in 11.8±7.0 hours, and 19% were admitted after 17.6±9.5 hours of observation. Three discharged patients returned within 7 days, 2 for uncomplicated recurrent AAF and 1 for chest pain subsequently found to be noncardiac in origin. There were no major complications attributable to the EDOU protocol. Conclusion: Selected patients with AAF for whom initial ED management fails can subsequently be managed in an EDOU with a high short-term conversion and discharge rate. [Koenig BO, Ross MA, Jackson RE. An emergency department observation unit protocol for acute-onset atrial fibrillation is feasible. Ann Emerg Med. April 2002;39:374-381.]