M mode and cross sectional echocardiography was carried out in three cases of cor triatriatum sinistrum (two infants and one adult). In two cases a peculiar double arch appearance, not previously reported, was found. All three cases were referred for surgery without cardiac catheterisation, and the diagnosis proved to be correct. The characteristic echocardiographic feature of cor triatriatum is an intra-atrial membrane detected in multiple planes of examination, curving anteroinferiorly and inserting some distance away from the mitral valve ring, proximal to the left atrial appendage. Superiorly the membrane runs parallel to, and a short distance behind, the aortic root creating a superior recess of the distal left atrial chamber. These features differentiate cor triatriatum from a supravalvar mitral ring. During diastole the membrane moves forward towards the mitral valve funnel. This, together with the arching appearance of the membrane on four chamber views and the more superior position of the membrane, makes it possible to distinguish cor triatriatum from total anomalous pulmonary venous drainage to the coronary sinus. From a review of past experience at the Brompton Hospital of the diagnostic accuracy of cardiac catheterisation in this condition, it is concluded that cross sectional echocardiography is superior to angiography as a technique for diagnosing cor triatriatum.