Patients with clinically asymptomatic, high-grade internal carotid artery stenosis (ICAS) often show cognitive impairments, such as memory dysfunction and attention deficits. However, it is still unclear whether these symptoms are caused by potentially reversible cerebral hypoperfusion or rather by a general unchangeable vascular damage. Here, 17 patients with one-sided high-grade, asymptomatic ICAS and 26 age-matched healthy controls underwent an MRI scan, including pCASL to assess brain perfusion, and a computer-based visual attention test. Patients show distinct unilateral cerebral hypoperfusion being significantly associated with contralateral attention deficits. Data indicate that chronic cerebral hypoperfusion in high-grade ICAS impairs cognitive function.