BACKGROUND: In neonates and young infants (less than 3 months), coarctation may be missed or underestimated by echocardiography, especially with a patent ductus arteriosus or severe concurrent illness. A reliable noninvasive screening tool for coarctation would be useful for these patients. METHODS: From 1997 to 2003, echocardiographic evaluation was performed in 63 consecutive patients with coarctation (47 neonates and 16 infants) as well as in 23 controls (16 neonates and 7 infants). End-systolic measurements were obtained from 12 different sites of the aortic arch. RESULTS: In patients, the diameters of the ascending and descending aorta were comparable to controls, but the dimensions of the transverse arch were significantly smaller. The distances between the origins of the great vessels were longer in patients with coarctation than in controls. The ratio of the aortic arch diameter at the left subclavian artery, to the distance between the left carotid artery and the left subclavian artery, which we propose as the carotid-subclavian artery index, was significantly smaller in patients with coarctation. A cut-off point at 1.5 showed a sensitivity of 97.7% and 94.7%, and a specificity of 92.3% and 100%, for neonates and young infants, respectively. The positive predictive value to have coarctation was 97.7% and 100%, for neonates and infants, respectively. CONCLUSIONS: The carotid-subclavian artery index is a simply obtainable noninvasive screening parameter, showing high sensitivity and specificity for coarctation, and may be useful in unstable patients or in those with a patent ductus arteriosus in which coarctation may be overlooked.