Extracorporeal membrane oxygenation (ECMO), a lifesaving procedure in neonates with severely impaired lung function, requires ligation of the right common carotid artery. In an investigation of the collateral pathways that maintain perfusion of the brain after right common carotid artery ligation, ten infants were studied with color Doppler imaging during and after ECMO. The circle of Willis was seen in all infants, and in every case there was retrograde flow in the distal internal carotid artery and the proximal (A1) segment of the anterior cerebral artery. Bilateral flow in the middle cerebral arteries and in distal branches of the anterior cerebral arteries was also seen, documenting flow to both cerebral hemispheres.