Review of the plain chest roentgenograms and angiocardiograms of 30 patients with Type 1 or Type 2 truncus arteriosus indicated that the diagnosis is suggested by cardiac enlargement, levocardia with right aortic arch, and increased pulmonary vasculature on the chest roentgenogram. Elevation of the left pulmonary artery and left atrial enlargement frequently are present and, when found, tend to substantiate the diagnosis. Exact preoperative diagnosis requires high-quality angiocardiograms. Even so, differentiation of Types 1 and 2 may be difficult. The truncal valve frequently is insufficient and often there is preferential regurgitation of blood into the right ventricle.