Patients with atrial fibrillation (AF) duration less than 48 hours and fulfilling inclusion and not exclusion criteria will be included in the study by a cardiologist on the day of cardioversion, and study nurse will be notified to plan for the investigations. The patient will undergo clinical examination, clinical history will be taken and blood sampling. Once 12 lead ECG, echocardiography, questionnaires, telemetry and MR have been done the patient will be electrically cardioverted After the cardioversion the patient will be subject to echocardiography of the heart again, blood-sampling, 12 lead ECG, and MR brain. The patient is discharged but will be studied by echocardiography of the heart and MR brain again on day 7 - 10. Thereafter the patient will be followed for 30 days - see flowchart.