To determine the relative position of the aorta and pulmonary artery in conventional chest roentgenograms, a correlative study with large roll film angiocardiograms was made in 70 cases of transposition of the great arteries, including 14 cases of double outlet right ventricle of the transposition type. We found 3 main types of interrelation between the great arteries: anteroposterior, oblique and side-by-side. From the interpretation of the conventional high kilovoltage roentgenogram it was possible to determine the position of the great arteries in the majority of the cases with levoventricular loop and left-sided aorta. In patients with dextroventricular loop and anteroposterior relationship between the great arteries, it was more difficult to identify the position of the arteries. Six exceptions to the loop rule occurred in 70 cases.