The aim of the study was to correlate cognitive decline and cerebral vasoreactivity in 150 asymptomatic right-handed patients with severe ≥70% unilateral internal carotid artery (ICA) stenosis and to evaluate the role of intracranial collateral circulation during cognitive testing. Cognitive assessment was performed by means of Montreal Cognitive Assessment (MoCA) and Mini Mental State Exam (MMSE) scales. Cerebrovascular reactivity (CVR) and intracranial collateral circulation were evaluated by means of breath holding index (BHI) and transcranial color Doppler (TCD) sonography. The results were compared with 150 right-handed controls matched for demographic variables and vascular risk factors. Patients with severe unilateral ICA stenosis had MMSE scores within a normal range, but MoCA scores were lower than normal. By examining the side of the observed stenosis, it has been noted that patients with left-sided ICA stenosis had lower MoCA scores in categories of language and episodic memory performance, while patients with right-sided ICA stenosis had lower MoCA scores in a category of visual-spatial skills. All patients had BHI values lower than normal. Subjects with a single intracranial collateral artery recruited had slightly better cognitive results than the patients with two or more collateral arteries activated. Results of the study showed that altered cerebrovascular reactivity and cerebral hypoperfusion might be responsible for the reduction of specific cognitive functions ipsilateral to the ICA stenosis, therefore BHI and MoCA might be useful tools when screening for cognitive decline in asymptomatic patients with severe ICA stenosis.