The St. Louis Carotid Occlusion Study demonstrated that ipsilateral increased O2 extraction fraction (OEF) (Stage II hemodynamic failure) measured by positron emission tomography (PET) is a powerful independent risk factor for subsequent stroke in patients with symptomatic complete carotid artery (CA) occlusion. The ipsilateral ischemic stroke rate at 2 years has been shown to be 5.3% in 42 patients with normal OEF and 26.5% in 39 patients with increased OEF (p = 0.004). In patients in whom hemispheric symptoms developed within 120 days, the 2-year ipsilateral stroke rates were 12% in 27 patients with normal OEF and 50% in 18 patients with increased OEF. Previous PET studies have demonstrated that anastomosis of the superficial temporal artery (STA) to a middle cerebral artery (MCA) cortical branch can restore OEF to normal. The authors discuss the undertaking of a study that will test the hypothesis that STA-MCA anastomosis, when combined with the best medical therapy, can reduce ipsilateral ischemic stroke by 40% at 2 years in patients with symptomatic internal CA occlusion and Stage II hemodynamic failure occurring within 120 days after surgery. Only patients with increased OEF distal to a symptomatic occluded CA will be randomized to surgery or medical treatment. The primary endpoint will be all strokes and death occurring between randomization and the 30-day postoperative cut off (with an equivalent period in the nonsurgical group), as well as subsequent ipsilateral ischemic stroke developing within 2 years. It is estimated that 186 patients will be required in each group. Assuming that 40% of PET scans will demonstrate increased OEF, this will require enrolling 930 clinically eligible individuals.