Morphological and histopathological aspects of aneurysms after patch aortoplasty for coarctation. Repair of coarctation of the aorta by synthetic patch grafting has been complicated by late aneurysm formation. These aneurysms differ macroscopically from atherosclerotic thoracic aortic dilatations. Specimens for microscopic examination were taken from 14 of 20 patients undergoing aneurysm resection. Histological analysis of the specimens showed medionecrosis in 13 patients of the specimens showed medionecrosis in 13 patients (93%), foreign body reaction in 11 patients (78%), and intimal thickening in 3 patients (21%). The three layers of the aortic wall could be identified in the aneurysms. On the basis of these results, we discuss the etiologic factors and pathogenetic mechanisms involved in the development of these aneurysms. 