The aim was to assess the value of continuous and pulsed wave Doppler ultrasound in the detection and differentiation of obstructive lesions of the aortic arch in neonates. In 31 neonates with proven arch obstruction (pre- or juxtaductal coarctation in 19 patients; postductal coarctation in five patients; interrupted aortic arch in four patients; aortic arch atresia in three patients), continuous wave Doppler interrogation of the descending aorta from the suprasternal notch revealed a high velocity jet (greater than 2.2 m/s) directed away from the transducer in 12 patients. Of these, four neonates had preductal coarctation, and five postductal coarctation. The remaining three patients had arch interruption or atresia. Image guided pulsed Doppler ultrasound recordings were obtained from the arch upstream from the obstruction, the descending aorta distal to the obstruction, and from the arterial duct. Patients with coarctation had a prominent diastolic flow directed away from the transducer in the arch upstream from the obstruction, representing a diastolic coarctation gradient, or diastolic steal either by the patent arterial duct or by collateral vessels. In contrast, patients with arch interruption or atresia had only a systolic flow signal in the proximal arch. Ductal flow was either bidirectional (preductal coarctation, arch interruption, arch atresia), continuous right to left flow from pulmonary artery to aorta (one case each of juxtaductal coarctation and arch atresia), or continuous left to right flow from aorta to pulmonary artery (postductal coarctation). In neonates wide patency of the duct often precludes the development of a large pressure drop across a coarctation. Conversely, a high velocity signal may be recorded from a patent but restrictive duct. In conjunction with imaging, pulsed Doppler velocity profiles from the arch and patent duct permit a meaningful interpretation of the haemodynamics of arch obstruction.