BACKGROUND A decrease in arterial compliance of the internal carotid artery has been associated with an increased risk in ipsilateral ischaemic stroke. However, so far, no technique has been validated to monitor the compliance of intracerebral arteries (Ca) in patients with carotid artery disease. In this study, we sought to monitor Ca in patients with unilateral symptomatic disease and to determine its variations during changes in PaCO(2). METHODS We studied 18 patients with unilateral symptomatic internal carotid artery stenosis >50% or occlusion. Patients underwent monitoring of arterial blood pressure (ABP) and middle cerebral artery cerebral blood flow velocities (CBFV) during baseline, hyperventilation and 5%CO(2) inhalation. Ca was calculated from pulsatile amplitudes of ABP and Cerebral arterial blood volume, extracted from the CBFV waveform using a new mathematical model. RESULTS At baseline, the decrease in Ca on the diseased side was correlated with the degree of stenosis (r = -0.35; P = 0.01). During hypocapnia, Ca was lower compared to baseline on the normal side (P = 0.004) and on the diseased side (P = 0.04). Ca reactivity, reflecting the changes in Ca per changes in 1 mmHg PaCO(2), was lower on the diseased side between baseline and hypocapnia (3.4 vs. 2.6%; P = 0.04). During hypercapnia, no changes in Ca on the diseased (P = 0.8) nor on the normal sides (P = 0.2) were observed. CONCLUSIONS The decrease in cerebral arterial compliance the side of stenosis/occlusion was correlated with the severity of the internal carotid artery disease. Further studies are needed to determine whether Ca may improve the prediction of ischaemic events in symptomatic and asymptomatic patients.