The measurement of cerebral blood flow (CBF) in addition to cerebral computerized tomography (CT) and angiography is most reliable in cases of transient ischemic attacks (TIA) and prolonged reversible ischemic neurologic deficits (PRIND). Alterations of CBF can be detected in symptom-free intervals. The cerebrovascular reactivity to CO2 stimulus is regarded as an especially suitable tool to prove the cerebrovascular reserve. If it is diminished, cerebral angiography should be carried out since it will often show major obstructive lesions. Angiography shows no sure correlation between CBF and collateral circulation. Strong opthalmic pathways in unilateral occlusion of the internal carotid artery (ICA) often coincide with compensated or only slightly alterated CBF and relatively small infarcts in CT. In about 70% of cases of ICA occlusion, CT shows an infarct mostly in region of the middle cerebral artery (MCA). Largest infarct volumes were found in the anterior area. Although resting CBF was normal in 55% of cases of unilateral ICA occlusion, CO2 reactivity was impaired in 68% of these Cases.