Right aortic arch is a rare anomaly, the diagnosis of which is seldom established during life. Paradoxically, it is the easiest of all the congenital anomalies of the circulatory system to recognize with certainty. This can be accomplished immediately in practically all cases in which a complete roentgenologic examination of the heart can be made. The importance of this becomes more manifest when we realize that the diagnosis cannot be established with any reliance by the usual clinical procedures. All cases familiar to the writer, which have been identified prior to the findings of the autopsy table, were diagnosed through the use of the roentgen ray. However, there is a strong likelihood that many cases of right aortic arch have passed through the hands of the roentgenologist without having been recognized. The factors responsible for this condition are not hard to find: (1) Elimination of fluoroscopy from the examination; (2) restriction of the radiographic examination to the postero-anterior film; (3)...