OBJECTIVES: Atrial fibrillation (AF) is the most common arrhythmic disease in Sweden. Persons with AF have a significant increased risk of stroke and the main treatment is therefore intended to prevent stroke by anticoagulation with warfarin or acetylsalicylic acid. However, new anticoagulation treatments will soon challenge the established treatments and there will be a need to know the cost of AF for cost-effectiveness comparisons. In this study we estimate the cost of AF in Sweden 2010.   METHODS: All relevant costs are identified, quantified and valued. The focus in the calculation is on the complications (ischemic- and hemorrhagic stroke, gastrointestinal bleedings and other major bleedings) caused by AF and treatments intended to lower the risks for ischemic stroke. A societal perspective is used, and therefore productivity loss caused by morbidity is included. Patients with CHADS2=1 or higher are included. RESULTS: In Sweden with 9,340,000 inhabitants, there are 118,000 patients with AF and at least one more risk factor of stroke, which comprise 1.26% of the population. Among the patients; 43.3% are treated with warfarin, 28.3% use acetylsalicylic acid and 28.3% are assumed to have no anticoagulation treatment. The cost of AF in Sweden is estimated at SEK 3.4 billion (€354 million and US$439 million) for 2010. This equals SEK 29,000 (€3,023 and US$3,745) per AF patient and year. The highest cost is caused by stroke, the second highest is the cost of monitoring the warfarin treatment. As the prevalence of AF is expected to increase, the AF cost is expected to rise in the future. CONCLUSIONS: AF causes high costs for the society. New treatments that could reduce the risk of stroke and be easily administered might have a potential of being cost-effective.