When atrial fibrillation (AF) causes such haemodynamic instability that a transfer of the patient to a hospital cannot be considered safe, and pharmacological management is not an option, cardioversion should be carried out without delay, even if the patient is not receiving therapeutic anticoagulation or has just eaten. When AF causes no haemodynamic instability and it has a clear and treatable underlying cause, e.g. myocardial infarction, myocarditis, hyperthyroidism or an acute pulmonary disease, the treatment should initially target the underlying cause and only after that the arrhythmia.