Background Lone atrial fibrillation (LAF) is defined by the presence of atrial fibrillation unassociated with other evidence of organic heart disease. There are conflicting data concerning the prognostic importance, rate of embolic complications, and survival in subjects affected by this arrhythmia. Methods and Results One hundred forty-five patients younger than 50 years at the time of the first diagnosis were identified; 96 had paroxysmal and 49 had chronic LAF. They were followed up with clinical and echocardiographic controls, and we recorded every thromboembolic complication and death. During the follow-up (10 ± 8 years) among patients with paroxysmal LAF, 1 (1%) had an ischemic stroke, 2 a transient ischemic attack, and 1 a myocardial infarction. In the group with chronic LAF, 1 patient had moderate heart failure, 2 myocardial infarction, and 1 transient ischemic attack. In this group, 8 embolic complications in 7 (16.3%) patients were observed. One patient with intestinal embolism died during surgery; 2 (6.1%) patients died suddenly. Conclusions The prognosis of young patients with paroxysmal LAF appears to be excellent, whereas patients with chronic LAF are at increased risk of embolic complications and higher mortality rates. Our results suggest that LAF is not always a benign disorder, as suggested by previous studies. Subgroups with substantially increased risk for thromboembolic events caused by LAF should be better identified. (Am Heart J 1999;137:686-91.)