The aim of this clinical study was to determine whether Doppler ultrasound can be used in newborn infants to assess the adequacy of palmar collateral circulation. Retrograde flow in the radial artery, distal to the site of manual occlusion of the vessel, was studied by Doppler technique. Forty-seven newborn infants, who underwent percutaneous radial artery cannulation were studied. Prior to cannulation palmar collateral circulation was tested in each infant, using the timed Allen test and was considered to be adequate. Pulsatile retrograde flow could be demonstrated in 11 out of 47 infants, but not in 36 of the 47 studied. During the period of cannulation none of the infants showed any sign of vascular insufficiency of the hand. It can be concluded that in newborn infants, the detection of pulsatile retrograde flow in the radial artery, using a Doppler flow meter, does not appear to have advantages over the Allen test as an indicator of adequate palmar collateral circulation.