Marked arteriosclerosislocalized to the descending thoracic and abdominal aorta was observed in a 19-year-old girl with interruption of the aorta (type A) and a ventricular septal defect. She died from rupture of saccular aneurysm of the pulmonary trunk. The descending thoracic and abdominal aorta showed marked thrombogenic, lamellar thickening of the intima. In contrast, no significant arteriosclerosis was detected in the ascending aorta. Several factors including diffuse sludging of blood, mural thrombosis and hemodynamicchanges, in close association with polycythemia, are hypothesized to be causative factors in the development of this lesion.