OBJECTIVE This study was undertaken to evaluate the progression of aortic aneurysms after patch aortoplasty repair of coarctation of the aorta. BACKGROUND Previous studies demonstrated a 5% to 25% incidence rate of repair site aneurysm 3 to 18 years after patch aortoplasty repair of coarctation. The natural history of aneurysmal progression in this disease entity has not previously been examined. METHODS Twenty-nine patients were identified 5.6 +/- 1 years (mean +/- SE) postoperatively and classified into two groups: Group A, aneurysm (n = 7); Group B, no aneurysm (n = 22). The presence of an aneurysm was defined angiographically as a ratio of the repair site diameter to diaphragmatic aortic diameter (aortic ratio) greater than or equal to 1.5. A 23% prevalence (7 of 29) of aortic aneurysm was identified. One patient in Group A underwent semiemergency aneurysmectomy and two patients in Group B were lost to follow-up. The remaining 26 patients were reevaluated 3 to 5 years later by clinical examination and chest radiography. Aortograms were performed in all patients with suspected aneurysm formation or progression. RESULTS Five of six patients in Group a demonstrated progressive aneurysmal dilation documented by an increase in aortic ratio from 1.64 +/- 0.06 to 2.04 +/- 0.2 (p = 0.03) and an increase in absolute aneurysm diameter from 2.5 +/- 0.3 to 3.6 +/- 0.5 cm (p = 0.006). Only 1 of 20 patients in Group B had evidence of new aneurysmal dilation (p less than 0.05 vs. Group A). Four patients in Group A have undergone elective aneurysmectomy, with equal distribution of true and pseudoaneurysms by pathologic examination. CONCLUSIONS Aortic aneurysm formation is common after patch aortoplasty repair of coarctation of the aorta. The majority of patients with an aortic ratio greater than or equal to 1.5 will show significant progressive aneurysmal dilation within 3 to 5 years.