This study was designed to assess the long-term effects of balloon angioplasty for coarctation of the aorta. Eleven asymptomatic children, aged 4 to 6 years, underwent balloon angioplasty. Mean peak gradient fell from 50.5 +/- 4.7 mm Hg before angioplasty to 21.7 +/- 3.1 immediately after angioplasty. Children were then followed up at 3 to 6 month intervals and were recatherized 5 to 14 months after balloon angioplasty. On the basis of these catheterization findings, patients were divided into three groups: group I--four patients, residual gradient less than 10 mm Hg and no anatomic abnormalities; Group II--three patients, increase of gradient to greater than 25 mm Hg, mean 34 mm Hg; Group III--four patients, aneurysmal dilatation in the area of the balloon angioplasty. The seven patients in groups II and III underwent elective resection of their coarctation at 7 to 28 months after balloon angioplasty with end-to-end anastomosis. Somatosensory evoked potentials were monitored during the operation. There were no operative deaths and no gradients between arm and leg pressures postoperatively. One patient had mild paresis of the lower extremities. Pathologic examination of the specimens revealed an absence of muscle and elastic lamella in the area of the aneurysms. This finding was present in all specimens regardless of whether there was aneurysmal dilatation. Neofibroelastic proliferation at the site of the tear was responsible for persistent gradients. Balloon angioplasty may result in aneurysmal formation and/or recurrent stenosis in the area of the tear necessitating elective surgical repair. Surgical treatment is the same as for native coarctation when done early after balloon angioplasty, but may be associated with increased risk because of the lack of collateral circulation. Continued follow-up of these lesions is necessary.