The AFFELECT -study compares two types of treatment modalities for acute atrial fibrillation or flutter for patients in whom rhythm control is desirable. The main purpose is to observe if these arrhythmias can be safely treated electively (within 5-9 days). All patients are recruited in the emergency department. Patients must be in good clinical condition so that they can be discharged regardless to which treatment modality is randomly selected to them. Patients randomized to conventional care are treated conventionally which means acute rhythm control is applied by electrical or medical cardioversion in the emergency department (within 48 hours of onset of the arrhythmia). Patients randomized to elective care are discharged immediately after adequate temporary rhythm control is assured. All patients will visit a cardiologist out-patient clinic at approximately one week after the emergency room visit. Patients randomized to elective treatment and still in atrial fibrillation or in atrial flutter will be restored to sinus rhythm by electrical or medical cardioversion at the out-patient clinic. Cardiovascular status and treatment options are evaluated for all patients. Anticoagulation is managed according existing guidelines for all patients. Due to possibility of delayed cardioversion in the interventional group (elective care group), all patients receive anticoagulation before the out-patient clinic despite their thromboembolic risk. All patients who have not received adequate anticoagulation for three weeks prior to the delayed cardioversion will undergo a transesophageal cardiac ultrasound to ensure they are not in excess risk for thromboembolic events. Patients randomized to elective treatment have the possibility to opt-out and undergo acute cardioversion if their symptoms are unmanageable during the first week before the out-patient clinical. All patients are monitored for their symptoms by a standardized quality-of-life questionnaire and for possibly required acute medical interventions during the first week and one month after the out-patient clinic. After one month, all patients undergo an electrocardiography (ECG) to ensure the maintenance of normal rhythm in both treatment groups. After the months follow-up all patients are subsequently monitored for a maximum of five years for need of medical interventions due to atrial fibrillation of atrial flutter. New antiarrhythmics such as flecainide are not prescribed during the first month.