Aortic aneurysm formation after coarctation repair is a serious and life-threatening complication. Repairs using synthetic materials such as Dacron^® may carry the highest risk of aneurysm formation and rupture. The authors sought to determine the prevalence of aneurysm formation in patients who previously underwent coarctation repair using Dacron^® patch aortoplasty at their institution. Between 1977 and 1994, 63 patients underwent isolated coarctation repair using Dacron^® patch aortoplasty. Aneurysms were defined as an aortic dimension 1.5 times that of the aorta at the level of the diaphragm as shown by angiography, computed tomography (CT) scan, or magnetic resonance imaging (MRI). Of 61 early survivors, 29 (47 %) experienced an aneurysm in the area of previous repair. Nine patients (31 %) had spontaneous rupture of the aneurysm, which caused death in seven cases. Elective or emergent aneurysm repair was performed for 20 patients without complication, and 2 patients are being monitored at this writing. The mean interval from patch placement to aneurysm repair was 15 years (range, 4–27 years). Overall freedom from the development of an aortic aneurysm was 97 % at 5 years, 90 % at 10 years, 69 % at 20 years, and 42 % at 25 years. After repair of coarctation using Dacron^® patch aortoplasty, the risk for aneurysm formation in the area of repair and death from rupture is extremely high. Therefore, in accordance with the 2008 American Heart Association/American College of Cardiology (AHA/ACC) guidelines, all patients with repaired aortic coarctation should undergo either CT or MRI imaging at least every 5 years to assess for aortic aneurysm formation. More frequent imaging should be obtained for patients previously repaired with Dacron^® patch aortoplasty.