To evaluate the predictive accuracy of two-dimensional echocardiography in the diagnosis of aortic arch obstruction, 540 consecutive patients aged 2 days to 15 years (mean 2 months) who underwent subsequent cardiac catheterization and angiography were prospectively studied. At angiography, 51 patients had aortic arch obstruction; of these, 35 had juxtaductal coarctation, 15 isthmic hypoplasia and 1 a type B interrupted aortic arch. The presence of arch obstruction was correctly identified with two-dimensional echocardiography in 45 of 51 patients with this condition (overall sensitivity 88%). Two-dimensional echocardiography clearly defined a juxtaductal coarctation in 33 of 35 patients and isthmic hypoplasia in 13 of 15 patients (sensitivity 94% and 73%, respectively). The form and type of interrupted aortic arch were clearly distinguished from other forms and types of arch obstruction. Among the 489 patients without aortic arch obstruction, two-dimensional echocardiography wrongly diagnosed the presence of such obstruction in 9 patients (overall specificity 98%). Forty-six (92%) of the 51 patients had at least one associated intracardiac abnormality. Twenty-two (44%) had a ventricular septal defect, 21 (42%) a bicuspid aortic valve and 4 (18%) subaortic stenosis. Five patients had complex congenital cardiac malformations. All associated abnormalities were prospectively identified with two-dimensional echocardiography. Thus, two-dimensional echocardiography is highly specific in diagnosing aortic arch obstruction. It is less sensitive for the diagnosis of isthmic hypoplasia in the neonatal period.