Episodes of atrial fibrillation longer than 48 hours are prone to thrombosis, and a few patients can thrombose with an incidental episode. Atrial fibrillation episodes longer than 48 hours have a significantly higher risk of thrombus formation, which may lead to arterial embolism, but there are marked differences in thrombogenic events between patients. Some patients can form thrombi from brief episodes, and some patients have prolonged AF without necessarily having thrombus generation. Patients with atrial fibrillation are prone to stagnant blood flow and poor circulation within the atria and are at a higher risk of thrombosis. Patients with atrial fibrillation should be assessed for risk of embolism, and those at high risk should be given anticoagulation therapy to avoid embolism in the arterial system. Atrial fibrillation should be seen in the hospital and anticoagulation should be given if necessary.