Although transesophageal echocardiographically derived parameters, notably spontaneous echocardiographic contrast (SEC) in the left atrium or left atrial appendage (LAA), are known predictors of embolism in atrial fibrillation, their value is less well known in patients who have lone atrial fibrillation (LAF). This study describes transesophageal echocardiographic findings and identifies clinical predictors of SEC in the left atrium or LAA in a cohort of patients who had LAF. Eighty-two patients who had LAF and 289 patients who had non-LAF and underwent transesophageal echocardiography were enrolled from July 1998 to March 2002. LAA abnormality was defined as the presence of an LAA area >5 cm(2), emptying or filling LAA velocities <25 cm/s, or the presence of SEC or thrombus in the left atrium or LAA; LAA abnormalities were significantly less frequent in patients who had LAF than in those who had non-LAF. SEC in the left atrium or LAA was significantly less frequent in patients who had LAF than in those who had non-LAF (29.3% vs 49.8%, respectively, p <0.001). In patients who had LAF, SEC in the left atrium or LAA was significantly (p <0.05) less frequent in patients who were </=60 years old (17.9%) than in patients who were >60 years old (39.5%) and in patients who had paroxysmal LAF (5.9%) than in those who had persistent LAF (45.8%). On multivariate analysis, age and persistent LAF were the only clinical variables independently associated with SEC. Thus, transesophageal echocardiography detects thromboembolism risk markers in patients who have LAF. These abnormalities are less frequent in patients who have LAF than in those who are at low risk and have non-LAF; however, in patients who have LAF, older age and persistent atrial fibrillation are associated with these risk markers.