Complete occlusion of the internal carotid artery is not necessarily associated with total disruption of cerebral blood flow distal to the site of the arterial obstruction. In fact, the flow of blood may be sufficient to maintain cerebral perfusion. This is accomplished by the shunting of blood from the external to the internal carotid artery by means of collateral circuits. While several major branch arteries of the external carotid artery may be recruited in this process, the ophthalmic artery is often the final common pathway before the anastomosis with the internal carotid artery. With the aim of elucidating these collateral circles, digital subtraction angiography was used to examine 100 patients with total occlusion of the internal carotid artery. These data were compared with those obtained using Doppler ultrasound, duplex scanning, and transcranial Doppler. Forty patients were excluded because there was no demonstrable collateral circulation, because the collateral circulation used the communicating arteries, or because the ophthalmic artery arose from the middle meningeal artery. Dynamic data were obtained by compressing the branches of the two external and common carotid arteries, and the blood vessels were identified by comparison with previous anatomic studies. We have constructed a precise map of the intra- and extracranial circles that connect the internal and external carotid arteries through the pathways that include the ophthalmic artery. This map demonstrates why apparently identical vascular occlusions may have divergent symptomatologies and clinical outcomes, which may range from the complete absence of symptoms to cerebral infarction.