Six patients who had been operated upon for coarctation of the aorta with patch grafting between 1958 and 1960 were followed for a minimum of 17 years. One of these patients received an aortic homograft patch for an abdominal coarctation and was in excellent health 21 years later. An aortogram showed no aneurysm formation. One other patient was reoperated upon 1 month after the primary operation because of a pseudoaneurysm. The remaining four patients received either a Dacron graft (three patients) or an Orlon graft (one patient), and they all developed an aneurysm in the aortic wall opposite the patch graft. Three of these patients successfully reoperated upon, but the fourth patient died 17 years after patch grafting because of rupture of the aneurysm. Microscopy (in three patients) showed varying degrees of degenerative changes in the aortic wall opposite the patch. There was no evidence of infection or foreign body reaction in these parts of the aortic walls. The reason aneurysms develop after patch grafting probably is that part of the circumference is replaced by a material with tensile characteristics differing from those of the aorta itself. Repair of aortic coarctations with synthetic patches therefore cannot be recommended.