Recent studies in the current era of combination antiretroviral therapy demonstrate an increased risk of cerebrovascular disease (CeVD) in HIV-infected individuals compared with uninfected controls, independent of traditional stroke risk factors. As the HIV-infected population ages, the clinical and public health impact of this elevated risk is predicted to grow. Despite the increased cerebrovascular risk, no tools exist to stratify HIV-infected individuals by their risk of stroke. We aim to evaluate the association between HIV infection and cerebral vasoreactivity (CVR), an estimate of cerebrovascular reserve capacity associated with ischemic stroke that can be measured non-invasively using inhaled carbon dioxide and transcranial Doppler ultrasonography. In this cross-sectional study, we will compare CVR between treated, virally suppressed HIV-infected participants and uninfected controls recruited from the SCOPE cohort and co-enrolled in Dr. Priscilla Hsue?s ongoing vascular studies, determining if HIV infection is independently associated with CVR. Furthermore, we will correlate CVR with flow-mediated vasodilation (FMD) of the brachial artery and carotid intima-media thickness (CIMT), measures of systemic endothelial dysfunction and atherosclerosis and established biomarkers of cardiovascular risk. The proposed investigation, the duration of which is expected to be one year, leverages the existing infrastructure of SCOPE and Dr. Hsue's studies, greatly maximizing the likelihood of success in achieving our aims. The findings from this study will build the foundation for a longitudinal study prospectively evaluating CVR as a biomarker for HIV-associated CeVD.