The purpose of this study is to present intermediate-term results of balloon angioplasty of native aortic coarctation in neonates and infants less than 1 year of age. During a 60-month-period that ended in January 1990, 19 infants ages 3 days to 12 months (median, 2.5 months), underwent balloon angioplasty of native coarctation with resultant reduction in peak-to-peak systolic pressure gradient from 39 +/- 12 mm Hg (mean +/- SD) to 11 +/- 7 mm Hg (p less than 0.001) and increase in coarctation segment size from 2.2 +/- 0.8 mm to 4.7 +/- 1.0 mm. None required immediate surgical intervention. Thirteen of the 19 (68%) had severe associated cardiac defects. There was one death (5%) 2 days after balloon angioplasty, and it was related to associated cardiac defect. One infant was lost to follow-up. It is too soon to restudy one infant. The remaining 16 infants had clinical (36 +/- 18 months) and catheterization (12 +/- 4 months) follow-up data. The residual coarctation gradient (22 +/- 15 mm Hg) and coarcted segment size (4.4 +/- 1.6 mm) remain improved (p less than 0.01) when compared with pre-balloon angioplasty values. Five of the 16 (31%) infants (four were neonates at the time of balloon angioplasty) had evidence for recoarctation (defined as gradient greater than 20 mm Hg) and underwent surgical resection (two) or repeat balloon angioplasty (three), all with success. None developed aneurysms.(ABSTRACT TRUNCATED AT 250 WORDS)