Twelve patients with asymptomatic occlusion of one (n = 8) or both (n = 4) internal carotid arteries were examined by positron emission tomography (PET) and transcranial Doppler ultrasound. PET measurements included the determination of the regional cerebral blood flow (rCBF), oxygen extraction ratio (rOER), cerebral metabolic rate of oxygen (rCMRO2), and cerebral metabolic rate of glucose consumption (rCMRGlc). Transcranial Doppler ultrasound (TCD) was used to determine the pathways and efficacy of collateralization via the circle of Willis and included spectrum analysis of flow velocities within the middle and anterior cerebral arteries as well as vasoreactivity tests. In correspondence with ultrasound evidence of a haemodynamically effective intracranial collateral circulation no significant differences between patients and controls were observed for rOER, rCMRO2 and rCMRGlc, but rCBF was globally reduced. Furthermore, in all patients with unilateral carotid occlusion PET excluded side asymmetries of any parameter studied. In contrast, flow velocity parameters measured by TCD were significantly reduced ipsilateral and significantly increased contralateral to the carotid obstruction. Vasodilative capacities, however, remained preserved even in the territory of the occluded carotid system. These data indicate that patients with asymptomatic carotid occlusion compensate by haemodynamic and not by metabolic mechanisms in contrast to symptomatic patients.