Cerebrovascular reactivity (CVR) is typically measured from changes in cerebral perfusion responsive to a hypercapnic gas challenge. Recent attempts to measure CVR using resting-state BOLD fMRI without gas challenge have utilized spontaneous fluctuations in breathing patterns. Since BOLD signal is an indirect measure of cerebral perfusion, a technique that enables cerebral blood flow (CBF)-based CVR under free-breathing conditions is highly desirable. Here, we used a real-time (17 fr/s) PC MRI to measure CBF-based CVR in the resting-state. We evaluated the feasibility of this new approach, and compared it with real-time PC MRI with gas-inhalation, and regular PC MRI with gas-inhalation.