Introduction: Left atrial thrombi are present in one third of patients with severe rheumatic mitral stenosis and atrial fibrillation. A left atrial mass can be diagnosed as a thrombus by transthoracic echocardiography in the presence of the predisposing factors for a thrombus such as mitral stenosis and atrial fibrillation. The sensitivity of transthoracic echocardiography for detecting left atrial appendage thrombi is low, and only a few case reports have been described in the literature. Case Presentation: This report describes the case of a 65-year-old female with severe rheumatic mitral stenosis and chronic atrial fibrillation presenting with congestive heart failure. This patient was shown, by means of transthoracic echocardiography, to have a large immobile left atrium cavity mass (6.5 × 3.4 × 2.3 cm) and a large left atrial appendage mass (1.7 cm) identified as a thrombus. Contrast computed tomography of the chest confirmed the diagnosis. Conclusions: Transthoracic echocardiography was sufficient to establish the diagnosis and to decide its management.Keywords:Atrial Function; Left Atrial Appendage; Thrombosis; Echocardiography Implication for health policy/practice/research/medical education: Better understanding of the use of transthoracic echocardiography in the differential diagnosis of cardiac masses.

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