The authors report the extremely rare occurrence of a congenital abdominal aortic aneurysm, 6 cm in diameter, found in a 1-month-old infant. Prenatal ultrasonography at 34 weeks' gestation had shown the aneurysm, which at that time was interpreted as a renal cyst. At operation, an 8-mm polytetrafluoroethylene tube graft was interposed between the infrarenal aorta and the bifurcation. Cardiopulmonary bypass facilitated operative management by permitting return of blood lost and by maintaining body temperature. In a review of the literature, the authors could not find any report of a neonatal aneurysm of this magnitude. Regrettably, the cause of this true aneurysm remains obscure.