Identifying high-risk patients with asymptomatic carotid stenosis (ACS) is necessary regardless of whether intensive medical therapy or aggressive treatment is applied. In order to assess the relationship between cerebrovascular reserve (CVR) measured by perfusion CT with inhalation of CO2 and the risk of ischemic events in ACS, this long-term follow-up study was conducted. Forty-five patients with ACS who underwent the examination of CVR measured by perfusion CT with inhalation of CO2 were collected and followed-up for at least 5 years. The primary end point was the occurrence of ipsilateral cerebral ischemic events. HRs and their 95% CI were calculated by Kaplan-Meier survival analysis and Cox regression models. The mean follow-up time was 68.7±10.7 months (40.0–84.0 months). 13 (28.9%) ipsilateral ischemic events were observed. The annual risk of ipsilateral ischemic events was 4.8%. Kaplan-Meier survival analysis and univariate Cox regression analysis indicated that patients with less CVR experienced more ischemic events (p=0.006 and p=0.013, respectively), which was confirmed by multiple Cox regression analysis (p=0.012). CVR measured by perfusion CT may potentially be the factor which can predict the risk of ipsilateral ischemic events in patients with ACS. Multidisciplinary management is necessary for these high-risk patients.