Thrombosis is a common complication of mitral stenosis, however the mitral regurgitation is considered as a factor reduces thrombus formation. Transesophageal echocardiography allows excellent determination of embolic source (left atrial or left atrial appendage). Objectives: Verify that remarks and discuss this phenomenon. Methods: A descriptive study conducted in 60 patients underwent transesophageal echocardiography for the mitral stenosis. Results: 1. Thrombus took 63,3 percent of study group. The odd ratio of pure mitral stenosis group was 15,27. The odd ratio of mild mitral regurgitation group was 16,85. 2. By transesophageal echq'lrdiography, he rate of thrombus was 63,3 percent. By transthoracic echocardiography, the rate of thrombus was 33,3 percent. Conclusions: 1. Transesophageal echocardiography is superior to transthoracic echocardiography to determine the source of embolism. 2. The mitral regurgitation is considered as a factor reduces the formation of clots.