One of the most striking features about subaortic stenosis is the diversity of lesions that it encompasses and the inconsistency of terms used in its classification. Authors use the same terminology to denote diVerent conditions and a variety of names for identical lesions. Some terms are descriptive, others histological or anatomical. Choi and Sullivan suggested a classification based on morphological features that can be determined on cross sectional echocardiography (table 1). Short segment obstruction is defined as subaortic stenosis with a length of less than one third of the aortic valve diameter, and consists of types previously termed membranous, diaphragm, discrete, fixed, fibrous or fibromuscular. The advantage of the term “short segment” is that it avoids descriptions that are inaccurate, and does not imply a histological diagnosis for that seen on echocardiography. Long segment subaortic obstruction is defined as stenosis that has a length of more than one third of the aortic valve diameter. This is sometimes called “tunnel” obstruction, but this term is confusing as it is applied to a number of diVerent conditions. Subaortic stenosis can also be caused by deviation or malalignment of structures in the left ventricular outflow tract in association with a ventricular septal defect or atrioventricular valve tissue in the subaortic area. Short segment obstruction is caused by a complex fibromuscular structure a short but variable distance below the aortic valve with extension onto the anterior leaflet of the mitral valve and occasionally to the right coronary cusp of the aortic valve. In addition to being the most common type of subaortic stenosis, controversy surrounds both its cause and management. A genetic predisposition has been suggested as there are reports of a familial incidence and a similar condition in Newfoundland dogs. Coarctation of the aorta, bicuspid aortic valve, mitral valve abnormalities or ventricular septal defect occur in more than 50% of patients 3 4 indicating that a congenital factor is involved in its pathogenesis. It is rare, however, in infancy and the characteristic changes seen on echocardiography develop gradually. 5 This suggests an acquired lesion based on a congenital or genetic predisposition. Gewillig and colleagues and others have suggested that abnormal flow patterns could result from a septal ridge, malalignment of the interventricular septum, a small or elongated outflow tract, or an apical muscle band. They postulated that turbulence could damage the endothelium, resulting in deposition of fibrin and the subsequent development of the typical fibromuscular obstruction. Controversy regarding the timing of surgery for patients with short segment subaortic stenosis revolves around a number of important considerations. These relate to the rate of progression of subaortic stenosis, the development and progression of aortic regurgitation, the risk of endocarditis, and the incidence of recurrence following resection. Cross sectional echocardiography can detect minor changes in the subaortic area of patients presenting with an asymptomatic murmur or another cardiac lesion. Early diagnosis, coupled with the low morbidity and mortality from surgery, has led to a number of authors 5 7 advocating early surgery to prevent increasing stenosis and possibly the development and progression of aortic regurgitation. It was also hoped that this would reduce the risk of endocarditis and the incidence of recurrence. Some authors have advised surgery at a systolic gradient of 25–30 mm Hg across the left ventricular outflow tract, 8 while other institutions developed a policy of operation at diagnosis regardless of the degree of obstruction or aortic valve involvement. 9 Results of this move to early surgery have only recently been evaluated. Two studies 9 specifically compared the outcome of patients who had early surgery with those who did not. These provided further information on the factors that need to be considered when deciding on the timing of surgery.