Discrete subvalvular aortic stenosis is a progressive lesion. Most reported cases have had a measurable left ventricular-aortic gradient that progressed in severity. This report describes 35 patients in whom no significant left ventricular-aortic obstruction was noted at initial cardiac catheterization, but who later were shown to have significant subvalvular aortic stenosis. In 24 of the 35 cases, absence of a significant left ventricular-aortic gradient (less than or equal to 10 mm Hg) was documented at initial cardiac catheterization. In 11 patients, a left ventricular-aortic pressure gradient was not obtained or not sought in the absence of clinical evidence of an obstructive lesion. In each case, discrete subaortic stenosis was not noted on angiography. Associated lesions included ventricular septal defect in 7, patent ductus arteriosus in 12, coarctation of the aorta in 8, pulmonary stenosis in 3, atrioventricular canal in 2 and miscellaneous lesions in 3 cases. The 35 patients had documentation of subvalvular aortic stenosis 3 months to 19 years after their initial study based on repeat catheterizations in 26, echocardiography in 6 or discovery at surgery in 3 cases. There were eight children with coarctation and no left ventricular-aortic gradient who developed significant subvalvular stenosis at a median of 2 years 9 months after initial cardiac catheterization. Of 30 patients with associated lesions, 23 had surgical intervention before development of subvalvular aortic stenosis, but only 5 of 17 patients with ventricular septal defect had surgical repair or palliation specifically for the interventricular communication before development of subvalvular aortic stenosis.(ABSTRACT TRUNCATED AT 250 WORDS)