The aim of this study was to investigate the influence of mitral regurgitation (MR) on left atrial (LA) thrombus formation and spontaneous echocardiographic contrast in patients with rheumatic mitral valve disease. LA thrombus and spontaneous contrast are considered risk factors for embolic complications. The presence of MR has been related to a low incidence of embolization; however, its effect on thrombus formation and spontaneous contrast has not been clarified. We studied by transesophageal echocardiography 55 patients with rheumatic mitral valve disease, who were receiving anticoagulant treatment. Atrial thrombus was detected in 13 patients who had a lower incidence of significant MR (p < 0.03), a smaller regurgitant jet (p < 0.02), and a higher incidence of atrial fibrillation (p < 0.05) than the rest of the group. Spontaneous contrast was detected in 34 patients with larger atria (p < 0.006), smaller regurgitant jets (p < 0.05), a smaller mitral valve area (p < 0.008), and a higher incidence of atrial fibrillation (p < 0.002) than the rest of the group. Patients without significant MR are at high risk for LA thrombus formation and subsequent embolization and represent a subgroup in whom careful anticoagulation is needed. Conversely, the presence of significant MR correlates with a lower incidence of spontaneous contrast, thrombi, and embolization.