The Extracranial-Intracranial (EC/IC) Bypass Procedure is a new method in order to try to increase the Intracranial Perfusion Pressure (ICPP). Postoperative patency of the bypass can be shown by means of Seldinger angiography. By this method no information however is obtained on the hemodynamic function of the bypass. OPG-Gee measures the Ophthalmic Artery Pressure (OAP) which is related to the ICPP. In this study OPG-Gee is performed without and subsequently with compression of the bypass. Quantification of the hemodynamic significance of the bypass is obtained by measuring the pressure decay in the ophthalmic arteries during this procedure. The Rest Pressure Difference (RPD), i.e. the pressure difference between the two ophthalmic arteries without bypass compression is measured. By subtracting the RPD from the BCD the remaining pressure is called the Net Bypass Pressure (NBP), i.e. the pressure difference, which the EC/IC bypass contributes to the ICPP. If, however, at constant systemic arterial blood pressure (SABP) the OAP drops during bypass compression on both sides, we hypothesize that the NBP is the sum of both drops, because, as a result of the interhemispheric steal mechanism, the less perfused hemisphere steals from the better perfused hemisphere. In a group of 40 patients OPG-Gee without and subsequently with bypass compression was applied. Twenty two of 40 OPG-Gee's showed NBP's of 4 mm of mercury or more (55%). In the group of 29 patients with unilateral internal carotid artery occlusion in 21 patients (72%) an NBP of 4 mm of mercury or more was found (mean of 13 mm of mercury).(ABSTRACT TRUNCATED AT 250 WORDS)