The role of the ophthalmic artery (OA) as a collateral to the cerebral circulation in carotid occlusive disease is somewhat controversial. The aim of this study was to assess the importance of this collateral by comparing the results of transorbital Doppler ultrasonography and regional cerebral blood flow measurements using SPECT and Xenon-133 inhalation in 41 patients with unilateral high-grade internal carotid artery occlusive disease. Both measurements were performed under basal conditions and after the intravenous administration of 1 g acetazolamide to test cerebral vasoreactivity. Based on the direction of OA flow, the patients could be divided into three groups. In Group 1 (n = 16) with anterograde OA flow before and after acetazolamide, baseline rCBF values did not differ significantly between the two sides, or from those of the controls. The side-to-side asymmetry of the response to acetazolamide was also normal. In Group 2 (n = 11) where the OA flow was anterograde or "0 flow" before, but became retrograde after acetazolamide, rCBF was significantly reduced on the symptomatic compared to the non-symptomatic side under basal conditions. However, the increase in rCBF after acetazolamide was within normal limits. In Group 3 (n = 14) the OA flow was reversed both under basal conditions and after the vasodilatory stimulus. Baseline rCBF was significantly lower (p < 0.05) on the symptomatic side compared to the non-symptomatic, and the asymmetry became even greater (p < 0.001) after acetazolamide. Our findings demonstrate that the OA may be an important collateral pathway in patients with ICA occlusive disease, and contribute to the cerebral perfusion reserve in the case of compromised blood supply.