BACKGROUND Cerebral vasomotor reactivity (VMR) represents an autoregulatory response of the arterial trunks on the specific vasoactive stimuli, most commonly CO2. OBJECTIVE The aim of this retrospective study was to compare VMR in high-grade symptomatic (SCAS) and asymptomatic carotid stenosis (ACAS), using the apnea test to evaluate the hemodynamic status. METHODS The study included 50 patients who were hospitalized at the neurology and vascular surgery departments as part of preparation for carotid endarterectomy. We evaluated VMR by calculating the breath holding index (BHI) in 34 patients with SCAS and 16 patients with ACAS, with isolated high-grade carotid stenosis. We evaluated the impact of risk factors and collateral circulation on BHI, as well as the correlation between the degree of carotid stenosis and BHI. RESULTS A pathological BHI was more frequent in the SCAS group (p<0.01). There was no difference in the range of BHI values between the groups, both ipsilaterally and contralaterally. Only male gender was associated with pathological BHI in both groups (p<0.05). Collateral circulation did not exist in over 60% of all subjects. We confirmed a negative correlation between the degree of carotid stenosis and BHI. CONCLUSIONS SCAS and ACAS patients present with different hemodynamics. While ACAS patients have stable hemodynamics, combination of hemodynamic and thromboembolic effects is characteristic of SCAS patients.