Background and Purpose: There are few data on the occurrence of microembolic signals (MES) in patients with lone atrial fibrillation (LAF). The aim of this work was to systematically study the frequency of IVIES in patients with symptomatic and asymptomatic LAF and to compare it with that of nonvalvular atrial fibrillation (NVAF). Methods: 37 consecutive acute stroke patients with LAF, 10 asymptomatic patients with LAF and 100 age-matched healthy controls were studied. Another 92 stroke patients with known NVAF were included in the study to compare patients with LAF regarding the presence of MES. Both middle cerebral arteries were monitored by transcranial Doppler ultrasound for at least 30 min at admission and after 1 week in symptomatic and asymptomatic patients with LAF. All patients with LAF were followed up for a mean duration of 18 months, and recurrent strokes were registered. Results: MES was detected in 11 (29%) symptomatic patients with LAF but only in 1 asymptomatic patient with LAF (chi (2) = 11.3; p = 0.0008) and in no control subjects (chi (2) = 106; p = 0.00001). There was no difference in the frequency of MES-positive patients and the number of MES between subjects with symptomatic LAF and known NVAF (29% in both groups; mean count, 16 +/- 4 vs. 17 +/- 6; p = 0.73). In patients with symptomatic LAF and NVAF who underwent anticoagulant therapy, there was no difference in the frequency of MES after 1 week of hospitalization (chi (2) = 1.53; p = 0.2). During a mean follow-up period of 18 months, 1 patient with symptomatic LAF and MES had a recurrent ischemic event 1 year after the first stroke and none of those with asymptomatic LAF had any events. Conclusion: Our study suggests that asymptomatic and paroxysmal LAF with a lower frequency of MES is a benign disorder compared to chronic and symptomatic LAF with a higher frequency of MES. Further studies need to justify whether MES has a predictive value in patients with chronic LAF who are prone to ischemic stroke. Copyright (C) 2001 S. Karger AG, Basel.