Health care resource utilization is high for patients presenting with acute atrial fibrillation (AF). The potential for treatment algorithms to safely reduce resource consumption in this setting has not been prospectively evaluated. We designed and implemented a practice guideline for the management of patients presenting to the emergency department (ED) with the primary diagnosis of AF, with emphasis on appropriate cardioversion, use of oral rate-controlling medications, and expedited referral to an outpatient AF clinic. We prospectively collected clinical and resource utilization data on all such patients for 14 months before and after institution of the guideline. Institution of the guideline was associated with a decreased rate of hospital admission (from 74% to 38%), with no differences in ED return visits or hospital readmission within 30 days. No strokes or deaths were observed. This large decrease in resource utilization during the intervention phase of the study translated to an average decrease in 30-day total direct health care costs of approximately $1,400 US dollars per patient. Our clinical and cost outcomes were minimally affected after statistical adjustment for baseline differences between study groups. We conclude that the implementation of our practice guideline was feasible, safe, and effective. Widespread adoption of such practices may have large financial implications for the health care system.