OBJECTIVE Some studies have suggested a link between impaired cerebral hemodynamics and stroke in patients with carotid stenosis. The aim of this study was to assess the effects of severe carotid stenosis on cerebrovascular reactivity and the possible changes after endarterectomy. METHODS Using bilateral transcranial Doppler ultrasound, we studied the changes of flow velocity after hypercapnia in the middle cerebral arteries of 10 control subjects and 24 patients (13 symptomatic and 11 asymptomatic) with unilateral severe carotid stenosis before and after endarterectomy. Cerebrovascular reactivity was evaluated with the breath-holding index (BHI). RESULTS Before endarterectomy, BHI (mean value +/- SD) was significantly lower (P < .001) in the stenotic side of symptomatic patients (0.40 +/- 0.2) than in control subjects (1.12 +/- 0.3), the stenotic side of asymptomatic patients (0.80 +/- 0.4), and the normal side of both symptomatic (0.93 +/- 0.3) and asymptomatic ultrasonics (1.03 +/- 0.2) patients. On the other hand, no significant difference in BHI was detected in control subjects, on the normal side of symptomatic and asymptomatic patients, and the stenotic side of asymptomatic patients. After endarterectomy, BHI significantly increased (P < .0001) on the stenotic side of symptomatic patients (1.06 +/- 0.2) while remaining substantially stable on the normal side of both symptomatic and asymptomatic patients and on the stenotic side of asymptomatic patients. CONCLUSIONS These findings suggest that the study of cerebrovascular reactivity to hypercapnia may be relevant in evaluating the functional effects of carotid stenosis. Further investigations are needed to confirm the reliability of the breath-holding method in the selection of patients who might benefit most from carotid endarterectomy.