Eleven patients (seven females and four males; age 4 to 24 years) with discrete subaortic stenosis (DSS) diagnosed on echocardiography were subjected to balloon dilatation. The site of the obstruction was 1 to 8 mm below the aortic valve. On the basis of echocardiographic appearance, the patients could be divided into three groups. Group I patients had a uniformly thin (1 to 3 mm) obstructing "membrane" (n = 7). Group II patients had a thin obstructing "membrane" present at the tip of a thick bulge from the interventricular septum (n = 2) (intermediate form). Group III patients had an obstruction caused by a thick ridge of tissue (6 to 8 mm thick, n = 2). Maximum inflatable diameter of the balloon used was less than or equal to the aortic valve anulus. After balloon dilatation in group I, the gradients across the obstruction fell from 86.6 +/- 16.9 mm Hg to 24.0 +/- 13.1 mm Hg. Relief of obstruction persisted on follow-up of 3 to 24 months (gradient 28.8 +/- 15.7 mm Hg). In group II patients gradients fell from 116 and 40 mm Hg to 58 and 20 mm Hg, respectively immediately after balloon dilatation. On follow-up of 6 and 9 months the gradients have increased to 76 and 32 mm Hg, respectively. In group III the gradients fell from 64 and 70 mm Hg to 15 and 16 mm Hg, respectively, immediately after balloon dilatation, which increased to 58 and 60 mm Hg, respectively, within 24 hours and persisted around that level at 3 months' follow-up.(ABSTRACT TRUNCATED AT 250 WORDS)