Twenty-five infants and children with native coarctation of the aorta had percutaneous balloon angioplasty over a 28-month period ending in May, 1987. The mean systolic pressure gradient across the coarctation decreased from 47.6 +/- 20.9 mm Hg to 10.3 +/- 7.3 mm Hg (p less than 0.001) following angioplasty, and the diameter of the coarcted segment increased from 3.2 +/- 1.7 mm to 7.8 +/- 3.5 mm (p less than 0.001). Clinical and echo-Doppler follow-up indicated excellent results in 16 of the 18 patients in whom 3- to 22-month follow-up was available; two infants required additional treatment (repeat angioplasty in one and surgical resection in the other). Fourteen patients who underwent repeat cardiac catheterization remain improved with regard to pressure gradient across the dilated coarctation (9.5 +/- 9.6 mm Hg, p less than 0.001) and angiographically measured sizes of the coarcted segment (10.3 +/- 3.2 mm, p less than 0.001). No aneurysm was seen in any child. We recommend balloon angioplasty as the therapeutic procedure of choice for relief of severe, previously unoperated coarctation of the aorta in neonates and young infants. Routine use of balloon angioplasty for unoperated coarctation of the aorta in children appears indicated, but should await longer follow-up results and reports of follow-up on a larger number of patients; this caution is mainly based on reports from other workers of aneurysm formation at the site of balloon dilatation.