A 55 year old man with bilateral internal carotid and unilateral vertebral artery occlusions presented subacutely with profound behavioural and cognitive changes featuring frontal lobe deficits. Neuropsychological testing showed severe cognitive impairment compatible with dementia. Anatomical imaging showed only a small right superior frontal infarction. Cerebral blood flow was severely reduced, with profound hypofrontality and limited hypercapnic reactivity, and cerebral metabolism was reduced primarily in the medial frontal lobes. After right sided extracranial to intracranial cerebral bypass surgery, both flow and metabolism improved, as did behavioural and neuropsychological deficits. Perfusion insufficiency from bilateral carotid occlusions, with secondarily reduced metabolism in the frontal zones bilaterally, may be an unusual cause of a reversible frontal dementia syndrome.