Salient clinical, hemodynamic, angiocardiographic, and pathologic findings are presented in 100 patients with truncus arteriosus communis, 79 of whom were studied at autopsy. In this study of typical truncus, all had a ventricular septal defect (type A). Truncus with a partially formed aorticopulmonary septum (type A1) was much the commonest form (50%). Cases with no remnant of aorticopulmonary septum (type A2) were second in frequency (21%). The distinction between types A1 and A2 could not be made with certainty in 9%, because these types merge into one another. Cases with absence of either pulmonary artery branch (type A3) were the least frequent form (8%). Truncus with interruption, atresia, preductal coarctation, or severe hypoplasia of the aortic arch (type A4) constituted 12%. The diagnosis of truncus is primarily angiocardiographic. The plane of the truncal valve in the lateral projection is distinctive. It tilts anteriorly, facing the patient's toes, which can be of assistance in differential diagnosis. The angiocardiographic features of type A4, although unfamiliar, are pathognomonic. Since the median age at death was only 5 weeks, and in view of the difficulties associated with pulmonary artery banding, our goal should be the surgical correction of truncus during the first and second months of life.