Resting cerebral blood flow (CBF) and the response to inhalation of 7 per cent CO2 was measured in 74 patients with symptoms of cerebrovascular disease. In order to evaluate their role in the identification of patients with significant arterial lesions, these measurements were correlated with the angiographic appearances, the clinical picture and the presence or absence of infarction on CT scan. Patients with carotid stenosis of 60 per cent or more had normal resting flows, but reduced responsiveness to CO2 inhalation. Patients with carotid occlusion had both reduced resting flow and reduced CO2 responsiveness. Infarcts were visible in 25 per cent of the hemispheres studied, and were more common in patients with fixed neurological deficits, but were also present in 17 per cent of patients with transient ischaemic attacks (TIAs). Reduction in the collateral supply from the contralateral carotid artery via the Circle of Willis further reduced CO2 responsiveness with ipsilateral internal carotid occlusion. The normal increase in CBF which occurs with the inhalation of carbon dioxide is diminished with increasingly severe bilateral disease, with infarction and with a fixed neurological deficit.