Congenital subaortic stenosis or developmental so-called "mid-systolic" stenosis poses a major hemodynamic problem for which a satisfactory surgical approach has yet to be developed. A review of recent case reports substantiates our impression that patients with this defect show little tolerance for major surgical trauma. Diagnosis is possible only through clinical suspicion. The most helpful features are evidence of left ventricular strain, atypical location of an aortic ejection murmur, absence of aortic valve calcification or post-stenotic dilatation by X-ray and absence of the anacrotic pulse wave. Diagnosis depends on demonstration of a subvalvular chamber by retrograde intraventricular pressure recordings or by contrast ventriculography.