Recurrences of atrial fibrillation in patients with embolic events lead the study of the left atrial appendage, where 90% of thrombus arelodged.To evaluate the function of the left atrial appendage in patients with atrial fibrillation.An observational study was done with the purpose of evaluating left atrial appendage function in patients with atrial fibrillation. Eighty onepatients were compared, 58 with atrial fibrillation and 23 with sinus rhythm. A transesophageal echocardiography was done to evaluate leftatrial appendage function by a conventional method and also by tissue Doppler.A thrombus was found in 22,4 per cent of the patients with atrial fibrillation. Flow velocity In and out of the appendix by pulsed Dopplerwere higher in patients in sinus rhythm (p<0,001). Mean velocity rates were reduced measured by tissue Doppler in patients with atrial fibrillation(p<0,05). In patients bearing thrombus, Tissue Doppler velocities were significant lower. Non significant correlations in velocitieswere found between both methods. The appendix area was larger (p=0.006) and ejection fraction was lower in patients with thrombus(p=0,003).Variables measuring function of the atrial appendage by both methods are useful for differing patients with atrial fibrillation from those insinus rhythm. Also both serve to detect those patients with a tendency towards thrombo-embolic events. The tissue Doppler characterizedthose patients with atrial fibrillation and thrombus in left atrial appendage.