Infants with severe aortic valve stenosis often are critically ill and require urgent surgical treatment. Currently, angiography is used at the time of cardiac catheterization to diagnose aortic valve stenosis. However, the use of this test may be hazardous in an unstable infant and may precipitate hemodynamic and clinical deterioration before surgery. Therefore, a noninvasive method of accurately making this diagnosis would be useful in that it would allow the risks of cardiac catheterization to be avoided. Between January 1982 and September 1983, 10 infants with critically severe aortic valve stenosis and intact ventricular septum were examined by echocardiography. There were no false-positive or negative results in this time period and several criteria for the noninvasive diagnosis of critical aortic valve stenosis were recognized. These included immobile aortic valve cusps and left ventricular hypertrophy with increased echo density of the left ventricular papillary muscles and mitral valve support apparatus. Patients without other aortic obstruction had poststenotic dilation of the ascending aorta, as evidenced by a ratio of the diameter of the ascending aorta to that of aortic valve anulus greater than 1.7. A disturbed Doppler velocity signal in the ascending aorta supported the presence of valvar stenosis. Nine patients underwent cardiac surgery and five survived. In five patients surgery was performed without angiographic examination. The correct diagnosis was made noninvasively in each, and four of the five patients survived surgery. Echocardiography was comparable to angiography in making the diagnosis and assessing the cardiovascular anatomic characteristics. Echocardiography could therefore replace angiography in selected infants with symptomatic aortic valve stenosis and should be routinely used in the evaluation of these patients.