Detailed assessments were carried out before and after operation in 33 consecutive patients with congenital fixed subvalvular aortic stenosis. Effective relief of obstruction to left ventricular outflow can be accomplished at minimal risk in patients with discrete subaortic stenosis, but a pressure gradient may persist after operation because of secondary hypertrophic obstruction which resolves with time. Tunnel subaortic stenosis is characterized by a diffusely narrowed left ventricular outflow tract. When an intraventricular pressure gradient was present in such patients, good but not complete relief of obstruction was achieved. In tunnel deformities with a gradient at the valve or annulus and in subaortic stenosis produced by mitral valve anomalies, operative intervention was ineffective.