The diagnosis of ostial stenosis of the left main coronary artery is usually made by use of coronary angiography. However, positioning of the catheter across the obstruction may obscure this diagnosis during contrast injection. Although a damping of arterial pressure when the catheter enters the left coronary artery may suggest ostial stenosis, it may not be possible to make this diagnosis with certainty during cardiac catheterization. We report a series of four patients in whom the left coronary ostium and proximal left coronary arteries were visualized by means of transesophageal echocardiography. Both ostial narrowing by plaque and abnormally fast flow velocities were seen. In each case the echocardiographic findings contributed to the subsequent management of the patients.