The temporal blood flow velocity profile in the descending aorta at the diaphragm was investigated using pulsed Doppler echocardiography in patients with documented coarctation of the aorta and in control patients. The ratio of peak systolic to peak diastolic frequency shift, an angle-independent index of pulsatility derived from the Doppler curve, proved to be an excellent indicator of aortic arch obstruction in the absence of a patent ductus arteriosus. Control groups included patients with a systemic right ventricle, systemic ventricular dysfunction and aortic stenosis, to determine if these conditions would confound the test. None of these factors were found to influence significantly the frequency shift ratio. However, the Doppler curves in six patients with interrupted aortic arch or severe coarctation, whose descending aorta was supplied by way of a large patent ductus arteriosus, were similar to those seen in control patients. Two indexes of wave transmission velocity, the rate of acceleration and the time to peak velocity, discriminated less effectively between patients with coarctation and the control patients, particularly those with aortic stenosis. After surgery for coarctation, the Doppler indexes and contour of the Doppler curve discriminated completely between patients with and without residual coarctation.