We used combined two-dimensional and pulsed Doppler echocardiography to examine 37 infants and children with coarctation of the aorta and 19 normal subjects. The ages ranged from 1 day to 16 years. We compared the differences between the Doppler flow signal from the ascending aorta and the descending aorta below the coarctation in each patient, as well as the differences between the corresponding flow signals in the aorta of patients with coarctation and normal subjects. Six variables were measured from each Doppler signal: acceleration slope (peak rate of acceleration), acceleration time (time from onset of flow to the peak systolic frequency), antegrade flow time, peak systolic frequency, peak velocity of flow, and deceleration slope (peak rate of deceleration). In patients with coarctation, each of these variables was significantly different in the descending aorta compared with the ascending aorta. There was a decrease in the acceleration slope (14 ± 13 vs 87 ± 67 kHz/sec) (mean ± SD), peak systolic frequency (1.8 ± 1.0 vs 5.2 ± 1.9 kHz), peak velocity of flow (0.70 + 0.40 vs 1.4 ± 0.44 m/sec), and deceleration slope (11 ± 11 vs 27 ± 12 kHz/sec). There was also a prolongation of the acceleration time (140 ± 50 vs 88 22 msec) and antegrade flow time (330 + 120 vs 270 ± 50 msec). In addition, these variables in the descending aorta of patients with coarctation were significantly different from those in the descending aorta of normal subjects. In eight patients with coarctation and a patent ductus arteriosus, there was no difference in the antegrade flow time between the descending and ascending aorta (240 + 64 vs 230 + 20 msec), whereas this was significantly prolonged in the descending aorta compared with the ascending aorta in patients with coarctation and no patent ductus arteriosus (350 + 120 vs 280 ± 52 msec). Pulsed Doppler ultrasound is a useful modality for evaluating patients with suspected coarctation of the aorta. In the presence of a patent ductus arteriosus with coarctation, the antegrade flow time is not prolonged but the other indexes remain abnormal. This may provide a means of assessing response to administration of prostaglandin in critically ill newborns with coarctation.