The clinical and echocardiographic variables related to left atrial spontaneous echo contrast were prospectively evaluated in a consecutive series of 400 patients undergoing transesophageal echocardiography with a 5-MHz single plane transducer. Left atrial spontaneous echo contrast was found in 75 patients (19%) and was significantly associated with atrial fibrillation, mitral stenosis, absence of mitral regurgitation, increased left atrial dimension and a history of suspected embolism. Seventy-one (95%) of the patients with spontaneous echo contrast had atrial fibrillation or mitral stenosis. Anticoagulant therapy had no significant association with spontaneous echo contrast. Multivariate analysis in 89 patients with mitral stenosis or mitral valve replacement showed that spontaneous echo contrast was the only independent predictor (p = 0.03) of left atrial thrombus or suspected embolism, or both. In 60 patients with atrial fibrillation of nonvalvular origin, spontaneous echo contrast (p = 0.01) and age (p = 0.03) were the only independent predictors of left atrial thrombus or suspected embolism, or both. It is concluded that left atrial spontaneous echo contrast is 1) a common finding in patients undergoing transesophageal echocardiography, 2) associated with conditions favoring stasis of left atrial blood, and 3) a marker of previous thromboembolism in patients with nonvalvular atrial fibrillation and those with mitral stenosis or mitral valve replacement.