From November 1996 until June 2000, we prospectively screened 438 adults who were in AF and had been admitted to our hospital for treatment with external electrical cardioversion. Patients with known or suspected coronary artery disease, organic valvular heart disease, cardiomyopathies, pericarditis, arterial hypertension (ie, > 140/90 mm Hg), myocarditis, conduction abnormalities, and thyroid disorders, or patients with uncertain duration of AF were excluded from the study. Patients with AF for > 48 h and < 6 months duration (data selected from previous ECGs in the patient’s records or from their attending physician), which was defined as having experienced a lone episode of AF were included in the study. Finally, 78 consecutive patients (mean [± SD] age, 59.3 ± 9.3 years) with AF and 10 apparently healthy volunteers (mean age, 59 ± 4.6 years) were enrolled into the study. All patients were treated conventionally with therapeutic anticoagulation (3 weeks before and 4 weeks after cardioversion; target international normalized ratio, 2.0 to 3.0), and all were receiving therapy with oral antiarrhythmic agents (sotalol, 40 patients; amiodarone, 38 patients). Antiarrhythmic treatment was started (after having reached the target international normalized ratio) from the time of diagnosis (at least 7 to 8 days before the scheduled cardioversion) by the referring physician and was continued for the entire observation period. Before cardioversion and after an initial transthoracic echocardiographic study was performed, a transesophageal echocardiographic study was performed, according to our hospital protocol. If thrombus16–17 or severe spontaneous echo contrast16,18 was present, the cardioversion was postponed (ie, the patient was not enrolled into the study). After successful conversion, the patients were followed up for 12 months. An ECG and clinical examination were performed at 24 h, and 3, 6, 9, and 12 months following conversion. This study was conducted according to the Helsinki Declaration and was approved by the ethics committee of our hospital, and all patients gave informed consent.