The purpose of this study is to determine the sensitivity and specificity of two dimensional echocardiography in the diagnosis of thrombosis of the left atrial appendage. Sixty patients with mitral rheumatic heart disease were examined prospectively 24 to 72 hours prior to cardiac surgery. Two images were used to identify thrombosis in the appendage: parasternal short axis at the level of the aortic valve with a lateral and superior inclination of the transducer and a modified apical five chamber view with counter clockwise rotation of the transducer between the apical long axis and the five chamber view. Diagnosis was corroborated during surgery and by histopathological analysis. In all cases the presence and predominance of mitral lesion (stenosis or regurgitation) were established by clinical history, electrocardiogram, chest roentgenogram and two dimensional echocardiogram. In 58 patients the lesions were also confirmed by cardiac catheterization. Of the 60 patients (46 females and 14 males between 16 and 61 years of age), eleven cases (18.3%) of left atrial thrombosis were detected, of which seven had formed in the left atrial appendage. All were confirmed during surgical intervention and pathological analysis. One thrombus in the left atrial appendage not diagnosed by echocardiography was found during surgery (Sensitivity:90.9%). In this case pathology studies demonstrated recently formed thrombi. In all cases in which two dimensional echocardiography did not show signs suggesting thrombosis, surgery confirmed that atrial thrombosis did not exist (Specificity: 100%). This study demonstrates the utility of two dimensional echocardiography in the diagnosis of thrombosis of the left atrial appendage, thus making it possible to schedule corrective surgery and the use of anticoagulants.