The syndrome of bilateral hemispheric border zone ischemia. Symptoms compatible with vertebrobasilar ischemia have been reported in patients with unilateral or bilateral carotid occlusive disease. Intracranial steal phenomena have been proposed to explain the symptoms. In a review of 54 patients with angiographically documented severe bilateral carotid stenosis (less than or equal to 2 mm residual lumen) or occlusion, eight had symptoms suggesting vertebrobasilar insufficiency. Five patients were identified retrospectively, and the other three were evaluated prospectively. Symptoms included various combinations of hemodynamically mediated, transient bilateral motor, sensory, or visual impairment. Dysarthria, dysphagia, and diplopia were generally absent. Each patient also described additional symptoms compatible with transient hemispheric or retinal ischemia. The anatomic regions subserving the bilateral vertebrobasilar-like symptoms could be correlated with angiographically estimated arterial border zones in both hemispheres and may thus represent bilateral hemispheric border zone ischemia rather than brain stem ischemia. An intracranial steal need not be invoked. 