The aim of this study was to assess the influence of mitral regurgitation on the prevalence of left atrial spontaneous echo contrast and thrombosis in 2,180 consecutive patients undergoing transthoracic and transoesophageal echocardiography. Two groups of patients were defined according to the absence (group I) or presence (group II) of grades 3 or 4 mitral regurgitation quantified by transoesophageal echocardiography. Group II was associated with a statistically significant lower frequency of spontaneous echo contrast (0.6 vs 11.2%; p < 0.0001), left atrial thrombosis (0.6 vs 4.2%; p < 0.03), ischaemic cerebrovascular accidents (1.2 vs 21%; p < 0.0001), transient ischaemic attacks (0 vs 12%; p < 0.0001) and systemic embolism (0 vs 4.6%; p < 0.01). Conversely, the prevalence of atrial fibrillation was higher in group II (28 vs 19%; p < 0.01) and there were more patients with left atrial dimensions > or = 5.5 cm (16 vs 6.7%; p < 0.0001). When mitral stenosis and valve prosthesis were excluded, there were no cases of spontaneous echo contrast (8.3 vs 0%; p < 0.001) or left atrial thrombosis (2.9 vs 0%; p < 0.05) in the group with grades 3 or 4 mitral regurgitation. The phenomenon of left atrial spontaneous echo contrast and/or thrombosis is rare in patients with grade 3 or 4 in native mitral valve regurgitation and explains the low incidence of systemic embolism in these cases.