Six infants with coarctation of the aorta underwent percutaneous balloon angioplasty over a 6-month period ending July 1985. These infants had associated cardiac defects including aortic stenosis, ventricular septal defect, and patent ductus arteriosus. Catheters used carried 5 to 10 mm balloons; 4 to 8 atm pressure was applied for 10 to 15 seconds, and the procedure was repeated at least three times. No significant complications were encountered during the procedure. The mean systolic pressure gradient across the coarctation decreased from 44.2 +/- 4.7 mm Hg to 11.7 +/- 9.4 mm Hg (P less than 0.001) after angioplasty, and the diameter of the coarcted segment increased from 2.9 +/- 0.7 mm to 6.3 +/- 1.2 (P less than 0.001). Long-term follow-up indicated excellent results in four patients; the other two infants required additional treatment (repeat angioplasty and surgical resection, respectively). No aneurysm was seen in any infant. Based on this experience and the reported high mortality and high recurrence rate after surgical repair in neonates and young infants, 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.