Background and Purpose—Patients with carotid artery occlusion (CAO) and ipsilateral transient ischemic attack (TIA) can have lasting cognitive impairment, despite the recovery of focal neurological deficits. We sought to assess whether cognitive impairment in these patients is associated with hemodynamic compromise and/or impaired cerebral metabolism.Methods—In 39 consecutive patients with a TIA associated with an angiographically proven occlusion of the carotid artery, we examined (1) cognitive functioning, (2) cerebrovascular reserve capacity of the middle cerebral artery ipsilateral to the CAO as measured by transcranial Doppler ultrasound, and (3) metabolic ratios as measured by1H-MR spectroscopy in the centrum semiovale ipsilateral to the symptomatic CAO. Findings were compared with those in healthy control subjects.Results—As a group, patients were cognitively impaired. Mean CO2reactivity and the mean ratio ofN-acetyl aspartate to creatine were decreased. In approximately one third of patients, lactate was present in noninfarcted regions. The presence of lactate proved to be a stronger correlate of cognitive impairment than MRI-detected lesions (&bgr;=0.41 versus &bgr;=0.15). Cognitive impairment did not correlate with CO2reactivity or the ratio ofN-acetyl aspartate to creatine.Conclusions—This exploratory study in patients with CAO and ipsilateral TIA showed that1H-MR spectroscopy–detected lactate in noninfarcted regions is a better indicator of cognitive impairment than MRI-detected lesions. Cognitive impairment did not correlate with CO2reactivity.