We performed duplex and transcranial Doppler studies in 36 patients with angiographically documented internal carotid artery occlusion (ICAO) to determine the effect of ophthalmic artery collateral (OAC) on measures of vascular resistivity both proximal and distal to the occlusion. Resistance in the common carotid artery, measured by the resistivity index, was significantly lower in the group with OAC than in those without OAC, indicating a shunt to the low resistance intracranial circuit. The pulsatility index (PI) of the Doppler signal in the ipsilateral middle cerebral artery, a measure of both inflow pressure and distal vascular resistance, did not differ between those with and without OAC. However, the presence of circle of Willis collateral pathways (anterior communicating and/or posterior communicating artery) did appear to have a significant effect on pulsatility. When both were present angiographically, PI was higher than in the group with only 1 Willisian collateral. These findings suggest that OAC has a marginal effect on vascular resistance in arterial bed distal to an ICAO, while Willisian collaterals appear to have a more important role in cerebral perfusion, as measured indirectly by Doppler methods.