To determine whether morphologic structures or abnormal flow patterns predispose to pathologic proliferation of subvalvular tissue, 26 patients (mean age 19.8 +/- 10.3 years) were studied greater than or equal to 6 months after operation for isolated discrete subvalvular aortic stenosis. The aortic root diameter and the mitral-aortic separation were measured with sector echocardiography. Flow patterns in the left ventricular outflow tract of these patients and control subjects were evaluated with a color flow mapping system optimized for the detection of turbulence. All control subjects had laminar flow throughout systole in the left ventricular outflow tract. By contrast, turbulence originating well below the site where the shelf had previously been resected was observed in 20 (77%) of the 26 patients. In 16 of these 20 patients turbulence was caused by a ridge, which in 13 patients could be identified as the offshoot of a ventricular band. In four patients the turbulence was caused by malalignment of the muscular and membranous septum, resulting in protrusion of the muscular septum into the outflow tract. Except for the latter four patients, the aortic root diameter was 84 +/- 10% of values predicted by body surface area, with values in six patients falling below the third percentile (p less than 0.01). The mitral-aortic separation was 9.7 +/- 3.5 mm, values in 21 patients falling above the 97th percentile (p less than 0.001). These data support the theory that discrete subvalvular aortic stenosis may be caused by a chronic flow disturbance, preferably in a small and long outflow tract. Left ventricular bands, if reaching the outflow tract, may be a factor.(ABSTRACT TRUNCATED AT 250 WORDS)