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The process of diagnosis by echocardiography of transposition of the great vessels is based fundamentally on the recognition of the position and relative orientation of the two ventricles and of the two vessels of the base of the heart. Traditional two-dimensional echocardiography allows us to establish the position of the left ventricle because of the atrio-ventricular (mitral) valve and its direct continuity with a sigmoid valve. If this continuity is absent, the mitral valve may be recognised because it closes after the tricuspid valve. It the tricuspid valve is on the right of and/or in front of the mitral valve, the aorta will in principle be found on the right of the pulmonary artery ("loop rule"). The orientation of the great vessels may then be determined. Two-dimensional systems are particularly useful in diagnosis of vascular disorders. If these are not available, traditional echocardiography may lead us to suspect this abnormality: 1. Provided that the orientation of the transducer is known at all times during the recording; 2. Provided the recording demonstrates simultaneously the two great vessels with their respective valves. Two other criteria are important if Van Praagh's rule or the "loop rule" is unhelpful: the diameter of the two vascular trunks, and identification of the valve by a study of the ejection times.