Objective: To determine whether extracranial-intracranial (EC-IC) bypass can improve cognition over 2 years compared to best medical therapy alone in patients with symptomatic internal carotid artery (ICA) occlusion and increased oxygen extraction fraction (OEF) on PET. Methods: Patients underwent 15O PET and were randomized if OEF ratio was .1.13 on the occluded side. Using blinded baseline and 2-year cognitive assessments, age-adjusted composite z scores were generated from subtests sensitive to right/left hemisphere plus global cognitive functioning. Multiple regression predicted 2-year cognitive change. Results: Eighty-nine patients were enrolled; 41 had increased OEF and were randomized. Two died, 2 were lost to follow-up, and 2 refused 2-year testing. Of the 35 remaining, ± had ipsilateral stroke or death, leaving 13 surgical and 16 medical patients. Controlling for age, education, and depression, there was no difference in 2-year cognitive change between the medical and surgical arms (95%confidence interval 20.5 to 0.5, p = 0.9). In post hoc analysis of 26 patients with no stroke in the follow-up period, cognitive improvement was associated with less impaired PET OEF at baseline (p = 0.045). Conclusion: Cognitive improvement following bypass surgery was not superior to medical therapy among patients with recently symptomatic carotid occlusion and increased OEF. Among those with no recurrent stroke, less hemodynamic impairment at baseline was associated with greater cognitive gain in both groups. Reversing cognitive impairment in hemodynamic failure remains an open challenge. Classification of evidence: This study provides Class II evidence that for patients with symptomatic ICA occlusion and increased OEF on PET, EC-IC bypass compared to no bypass does not improve cognitive function after 2 years.© 2014 American Academy of Neurology.