AbstractModern techniques of cardiopulmonary bypass combined with methods for myocardial protection make the conventional operations of prosthetic patch aortoplasty, aortic valvotomy, and transvalvular resection of subaortic membranes dependable and accurate procedures which can provide the obliteration or successful reduction of systolic gradients with rare mortality or morbidity. Sonography provides a technique for the diagnosis of critical aortic stenosis in the newborn, permitting expeditious surgical intervention, while prostaglandin therapy provides a technique for the restoration of systemic blood flow and the reversal of acidemia in preparation for surgical intervention. Hypothermic techniques provide protection of ischemic myocardium which is thought to be a principal cause for earlier failures in the management of critical aortic stenosis. An appreciation that critical aortic stenosis need not be cured but only relieved for infant survival has led to the application of dilatation techniques using mechanical dilators and angioplasty balloon catheters. Successful repair of such complex malformations as atrioventricular septal defect, aortic arch interruptions, and the coarctation syndrome in the neonate has uncovered a variety of new fibrous, muscular, and fibromuscular left ventricular outflow tract obstructions which appear to be acquired and progressive and not solely congenitally determined. Sonography provides a technique for the detection and the serial assessments of these lesions. Radical operative approaches have been devised for enlarging, reconstructing, exposing, and replacing the left ventricular outflow tract. These operations, which have thus far found their principal use for the relief of complex and combined left ventricular outflow tract obstructions in older children and for reoperations on the left ventricular outflow tract, will achieve a fuller potential when the procedures are miniaturized and adapted to the use in surgery of the neonate and young infants.