The human heart is strikingly asymmetrical along the left-right body axis. If one begins with the position of the heart in the left chest, continues through asymmetrical venous drainage into the atria and asymmetrical orientation of the 2 anatomically and functionally distinct ventricles, and finally proceeds through the highly asymmetrical coil of semilunar valves and great vessels, the structure and function of the human heart are precisely aligned to the left-right axis. When cardiac asymmetries either fail to develop or align incorrectly relative to each other or relative to other organs, a plethora of congenital heart disease results. This group of heart diseases is called heterotaxy syndrome and represents both a difficult clinical challenge and a fascinating window into the biology underlying one of the most fundamental embryological processes, namely, the mechanism by which an organism establishes the 3 body axes. Positioning of organs along the left-right axis can be divided into 3 broad classes: situs solitus, in which all organs are positioned normally; situs inversus, in which there is mirror image reversal of all organs; and heterotaxy, in which there is any positioning of organs along the left-right axis differing from situs solitus and situs inversus (Figure). Pure situs inversus is found in 1 of 8500 in the general population and is usually not associated with intracardiac defects. In contrast, heterotaxy has a high degree of association with intracardiac defects. It has a reported incidence of 1 of 10 000 and is associated with at least 3% of cases of congenital heart disease.1 The anatomic spectrum of organ laterality. A, Situs solitus. B, Right atrial isomerism. The liver is midline, there are 2 eparterial bronchi, the position of the stomach and cardiac apex is indeterminate, and there is asplenia. C, Left atrial isomerism. The liver is midline, there …