Despite numerous epidemiologic studies, few data exist on recent trends in admissions to United States hospitals of patients with atrial fibrillation (AF) as a primary diagnosis and on the determinants of outcome of these hospitalizations. In the present study, we investigated recent trends in the incidence of AF admissions to United States hospitals from 1996 to 2001 using a representative sample of all acute-care hospital admissions in the United States provided by the Centers for Disease Control, with special attention to the importance of age, gender, race, and other determinants of outcome, namely, in-hospital mortality.