Obstructive sleep apnea (OSA) is a chronic disorder caused by intermittent obstruction of the upper airways during sleep. OSA patients are prone to cardiovascular disease and stroke. Cerebrovascular reactivity (CVR) is an index to assess the degree of impairment of cerebrovascular regulation. Here, a breath-hold index (BHI) was introduced as a surrogate for CVR to evaluate subjects with OSA and their controls. Preliminary results from an ongoing study found BHI to be significantly elevated in OSA for both BOLD based regional, and global CBF. The results agree with a recent MRI-based CVR study using an exogenously administered hypercapnia stimulus.