OBJECTIVES: Atrial fibrillation (AF) remains a major health problem, affecting 2.2 million adults in the US with an estimated cost burden of $3 billion annually. Treatment approaches vary significantly and can include both interventional and pharmacologic options. The purpose of this research was to characterize the clinical and economic impact associated with AF. METHODS: Patients with continuous health benefit eligibility and a “new” diagnosis of AF (ICD-9-CM 427.3x) were identified from a large, geographically diverse administrative claims database in the US (N=43 million) between 1 January 2000 and 31 December 2001 with a variable follow-up period of ³ 6 months. Warfarin-treated (WT) (N=6,846) and untreated (UT) (N=40,849) cohorts were evaluated based on initial therapy associated with the AF diagnosis. Study outcomes included patient demography, treatment patterns and direct medical costs. RESULTS: The AF cohorts were comprised of 58.5% males with a mean age of 57.9 ± 15.8 yrs; mean duration of follow-up was ~20 months. WT was initially received by 14% of this AF cohort with only 37% receiving warfarin sometime during follow-up. The WT cohort spent approximately 34% of their study follow-up exposed to warfarin (average number of days on therapy = 206.5; average study duration = 608.7 days). Annualized AF-related costs were $10,560 ± 290 (WT) versus $10,131 ± 143 (UT). CONCLUSIONS: Significant proportions of AF patients do not receive pharmacotherapy. Furthermore, pharmacotherapy appeared to be sub-optimal among those who received it. Better management of AF pharmacotherapy or use of newer therapies that provide appropriate anticoagulation are necessary to reduce the burden associated with this costly disease.