The atrial fibrillation is linked to an overmortality (x2), except in patients with lone atrial fibrillation. The severity of atrial fibrillation is due to the risk of thromboembolism event, especially stroke. This risk is very high in presence of rhumatismal mitral valve pathology or prosthetic valve. The annual risk of thromboembolism has been evaluated and the CHADS2 score defined from 0 to 6 In function of the CHADS2 score the risk of thromboembolism event is 1.9 to 18.2%/year. Other factors of thromboembolism accident are actually evaluated such as renal insufficiency, proteinuria. The main objective of atrial fibrillation treatment is the prevention of thromboembolism event. The antivitamin K agents proved their efficiency in term of mortality and morbidity justifying to identify the patients at risk of thromboembolism event.