Ten children, 10 weeks to 18 years old, with discrete subaortic stenosis (DSS) underwent transluminal balloon dilatation of the subaortic obstruction. The DSS was visualized by 2-dimensional echocardiography and cineangiography. Six patients had a thin discrete "membrane" immediately below the aortic valve (group I) and 4 patients had a thicker fibromuscular ring about 1 cm below the aortic valve (group II). In group I, the mean gradient decreased from 82 +/- 49 mm Hg (range 35 to 164) to 22 +/- 15 mm Hg (range 5 to 40); in group II, it decreased from 155 +/- 18 mm Hg (range 132 to 177) to 85 +/- 44 mm Hg (range 60 to 150). Three patients had follow-up cardiac catheterization 1 year later. Their mean gradient soon after the procedure was 37 +/- 23 mm Hg. On follow-up, it was still 37 +/- 19 mm Hg, indicating persistence of relief of the obstruction. Because of the high residual gradient in group II, 3 patients had surgical relief of the obstruction. The degree of aortic regurgitation present before the dilatation in all 10 patients did not change after the procedure. The mechanism of relief of the obstruction was by tearing of the subaortic membrane. Our data suggest that relief of subaortic obstruction is more favorable in the thin, membranous DSS.