A 67-year-old woman with a recent history of recurrent ischemic stroke secondary to right vertebral artery stenosis suffered acute onset of left homonymous hemianopsia and the medial longitudinal fasciculus syndrome, which resolved with hyperdynamic therapy. However, consciousness deteriorated 6 hours later. Perfusion computed tomography (CT) revealed regions of prolonged mean transit time in the bilateral cerebellar hemispheres, vermis, brainstem, and right occipital lobe, which were more extensive than the ischemic lesions demonstrated by diffusion-weighted magnetic resonance (MR) imaging. Subsequent cerebral angiography showed occlusion of the right vertebral artery. The patient underwent emergent left superficial temporal artery to left superior cerebellar artery bypass. Postoperatively she demonstrated resolution of the preoperative perfusion CT/diffusion-weighted MR imaging mismatch and improved neurological deficits. Early revascularization in a patient with perfusion CT/diffusion-weighted MR imaging mismatch following acute vertebrobasilar stroke can lead to improvement in cerebral perfusion and neurological function.