To the Editor.— Doppler ultrasound assessment of flow velocity in cerebral vessels in neonates has generated intense interest since the technique was originally applied to the preterm infant by Bada et al.1 The value and indication both as a research tool and useful clinical tool have yet to be defined. We have been using a range-gated, pulsed Doppler instrument with a specially designed transducer for the past 18 months and would like to add some comments to those of Bejar et al2 and Volpe et al3 concerning Doppler ultrasound, pulsatility index, patent ductus arteriosus effects on cerebral flow, and autoregulation.