Sixteen patients with vascular lesions of the vertebral and basilar arteries were studied with magnetic resonance imaging. The vascular abnormalities included seven cases of atherosclerotic disease with partial or complete thrombosis, six aneurysms, two cases of vertebrobasilar dolichoectasia, and one basilar artery dissection. Magnetic resonance effectively demonstrated vascular thrombosis with occlusion as high signal intensity on spin echo (SE) sequences with absence of flow void and no flow enhancement on gradient echo (GRE) images. Nonthrombosed aneurysms exhibited mixed signal intensity on SE images and hyperintensity on GRE images. Intraluminal thrombus also appeared heterogeneous, with variable signal intensity depending on the specific components of hemorrhage present. Correlation of the SE and GRE scans was helpful for distinguishing stasis and turbulent flow from thrombus. Compression of cranial nerves by vascular structures was clearly depicted in both cases of dolichoectasia. Basilar artery dissection was displayed as a focal area of flow void surrounded by a thrombosed false lumen. Magnetic resonance is an effective noninvasive method for evaluating vascular pathology of the vertebrobasilar system.