Forty patients operated on for fixed subvalvular aortic stenosis underwent cardiac catheterization preoperatively, immediately after coming off cardiopulmonary bypass and at long-term (1 to 14 years later, average 7 +/- 3.9 years). The age range was 3 to 50 years (average 15 +/- 12 years) with 27 (68%) aged under 18 years. Twenty-seven patients were male. The stenosis was the thin membranous type in 29, the fibromuscular collar type in 5, the tunnel type in 5 others and related to supernumerary mitral tissue in the remaining patient. Significant other pathology was associated in 13 cases. In addition to excision of the membrane or the fibromuscular ring, the surgeons performed myotomy in 6 cases, myomectomy in 12 cases, large resection of muscular and fibrous tissue in tunnels, and aortic valve replacement in 3 cases. There was no operative fatality. Permanent cardiac pacing was required in 1 patient for complete atrioventricular block. The peak systolic pressure gradient fell from 87 +/- 32 to 31 +/- 10 mmHg (p < 0.0001) at the immediate control: it remained > 30 and even 50 mmHg in 3 patients (7.5%), 2 of whom had tunnel types and the other the supernumerary mitral tissue. The gradient increased in the long-term to 42 +/- 11 mmHg, 1 patient with a membrane developed a gradient of 40 mmHg and 4 others (10%) developed a gradient > 50 mmHg (3 tunnels and 1 membrane). The 5 patients with tunnel types either had a residual stenosis or restenosis and underwent aorto-ventriculoplasty by Konno's procedure 1 to 8 years later. This operation should be the procedure of first intention, even in small children: the large resection is only acceptable when it cannot be performed or when aortic ring hypoplasia is mild. There is no residual stenosis and restenosis is rare (2.5%) in the membranous and fibromuscular types, probably because of the widespread use of myotomy and myomectomy. In the absence of severe associated malformations, surgery in only justified when peak systolic pressure gradients are > or = 50 mmHg.