It is a known fact that the value of the cerebral blood flow (CBF) in the hemisphere where the internal carotid artery (ICA) is occluded may serve as a quantitative parameter of the collateral blood supply (CBS). Comparison of the findings obtained by cerebral panangiography (CPAG) versus inhalational studies of the CBF in 38 patients with occluded ICA has demonstrated that although CPAG is quite adequate for assessing the status of the collateral blood circulation routes, a more optimal method for evaluating the status of the CBS is measurement of the CBF since in at least half the cases CPAG yields inaccurate information about the CBS in patients with impaired cerebral blood circulation. The studied methods are not competitive but should be used in conjunction for obtaining the most complete information about the cerebral hemodynamics which is particularly important to consider in making decisions about the advisability of surgical intervention.