OBJECTIVES: Atrial fibrillation (AF) is the most common cardiac arrhythmia. The prevalence among adults is estimated to be ≥1% in Sweden, increasing with age to nearly 10% among those over 80. The proportion of elderly will rise in coming years resulting in a higher prevalence of AF and probably increasing costs to the Swedish society. The aim of the present study was to estimate the annual cost of AF from a Swedish perspective. In addition, validity of data and problems regarding diagnosis coding were evaluated. METHODS: Prevalence-based cost-of-illness analysis of AF based on year 2007. Direct medical and non-medical, as well as indirect costs, were considered. Data were based on information from the literature, Swedish registries, and an expert panel. RESULTS: The annual total cost of AF was estimated to be SEK6600 million (approximately €600 million); direct medical costs 84%, direct non-medical costs 4%, and indirect costs 12%. The key driver (55%) of the result was the direct cost of heart failure and stroke, both having developed as a consequence of AF. Data from the registries were somewhat difficult to interpret due to possible miscoding or ambiguity in data inputs. In particular there was uncertainty associated with the coding of primary and secondary diagnosis as the cause-and-effect relationship varies between patients. CONCLUSIONS: This is the first comprehensive analysis of the direct and indirect costs of AF and its main complications in Sweden. The results showed the annual cost of AF is very high, but it is still likely to be underestimated as a conservative approach was applied in the analysis. This suggests further research is needed.