A consecutive series of 80 patients with atrial fibrillation were studied with both precordial and transoesophageal echocardiography. Left atrial spontaneous contrast echoes were observed in one patient with precordial echocardiography and in 26 patients (33%) with transoesophageal echocardiography. They were found most commonly in patients with rheumatic mitral valve disease (67%) but were observed in 28% of patients with lone atrial fibrillation. Their presence was unrelated to the age, gender and therapy of the patient. Although they were more common in patients with a large left atrium, they were sometimes observed in a normal sized atrial chamber. They were more common in chronic (40%) than in paroxysmal atrial fibrillation (5.6%). No patient had severe mitral regurgitation, but contrast echoes were observed in some patients with mild or moderate mitral regurgitation. Of the 26 patients with spontaneous contrast echoes, six (23%) had echoes consistent with left atrial thrombus compared to one of the 54 patients without these echoes (1.9%) (P = 0.006); 17 (65%) had suffered a previous thromboembolic event compared to 17 of the 54 without these echoes (32%) (P = 0.009). These data support the concept that spontaneous contrast echoes in the left atrium are associated with sluggish blood flow and a thrombogenic environment. Transoesophageal echocardiography may thus be useful in assessing which patients with atrial fibrillation might most benefit from anticoagulation. This hypothesis needs to be evaluated further in a prospective study.