Sebok M, van Niftrik CHB, Winklhofer S, et al. - This study was conducted to evaluate mapping cerebrovascular reactivity impairment in patients with symptomatic unilateral carotid artery disease. Between February 2015 and July 2020, a prospective cohort study was performed at the Clinical Neuroscience Center of the University Hospital Zurich, Zurich, Switzerland. Researchers recruited a total of 102 patients with newly diagnosed symptomatic unilateral internal carotid artery (ICA) occlusion or with 70% to 99% ICA stenosis. An age‐matched healthy cohort of 12 individuals had undergone an identical blood oxygenation‐level dependent (BOLD) functional magnetic resonance imaging assessment. Cerebrovascular reactivity (CVR) impairment was evaluated using BOLD functional magnetic resonance imaging with a standardized CO2 stimulus. In this study, 67 individuals had unilateral ICA occlusion and 35 patients unilateral 70% to 99% ICA stenosis. The findings revealed that comprehensive BOLD‐CVR mapping allows for identification of hemodynamically relevant symptomatic unilateral carotid artery stenosis or occlusion.