Partial or complete absence of the thymus (DiGeorge syndrome, III-IV pharyngeal pouch syndrome) is often associated with agenesis or hypoplasia of the parathyroid glands and, almost invariably, with cardiovascular malformations. The clinical and pathologcial findings in 10 cases proven at necropsy are presented. All patients presented with cardiac symptoms and signs in the first weeks of life and, with one exception, all died of a cardiac cause. Major cardiovascular malformations were found in all 10 cases. Four had, in association with a ventricular septal defect of the infundibular type, an interrupted aortic arch, which was left-sided in two and right-sided in two other cases. Four patients had truncus arteriosus type I, in two of them associated with a right-sided aortic arch. Two patients with tetralogy of Fallot had a right-sided aortic arch. Only two of the 10 had a normally developed left aortic arch. Aberrant subclavian arteries were found in five cases. From our observations and a survey of the previously published patients it appears that 90 per cent of the necropsy-proven cases of DiGeorge syndrome have cardiovascular malformations and that 95 per cent of these malformations can be classified as aortic arch anomalies, truncus ateriosus, or tetralogy of Fallot.