The incidence of spontaneous echo contrast in the left atrium, the factors with which it is associated, and its clinical significance were studied in 128 patients with native mitral valve disease or mitral valve replacement. All patients underwent both transesophageal and transthoracic echocardiography. Spontaneous echo contrast was visualized in 42 patients (33%) during transesophageal and in none during transthoracic echocardiography. Patients with spontaneous echo contrast had a significantly larger left atrial diameter (6.1 [1.1] cm vs 4.9 [0.9] cm, p < 0.001) and a greater incidence of both atrial fibrillation (54% vs 4%, p < 0.001) and left atrial thrombi (60% vs 28%, p < 0.01) compared with patients without spontaneous echo contrast. Multivariant analysis confirmed that these factors were independently associated with spontaneous echo contrast (left atrial size, p < 0.001; atrial fibrillation, p < 0.001; left atrial thrombus, p < 0.01). Patients with pure mitral regurgitation did not have any spontaneous echo contrast and anticoagulation did not influence its incidence. Spontaneous echo contrast was the only factor that was associated with a previous history of systemic embolization (history of systemic emboli in 28.6% of patients with spontaneous echo contrast vs 13.9% without spontaneous echo contrast, p < 0.05). In conclusion, spontaneous echo contrast is a common finding in the left atrium of patients with mitral valve disease or mitral valve replacement and represents a marker of increased thromboembolic risk in these patients.