Angiographically occult vascular malformations (AOVM) are cerebrovascular malformations that are not demonstrable on cerebral angiography. The majority of AOVMs located in the third ventricle are cavernous malformations. To the best of our knowledge an angiographically occult arteriovenous malformation (AOAVM) of the third ventricle has not been previously reported. We report an unusual vascular malformation of the third ventricle presenting with hydrocephalus due to mass effect, verified histopathologically as an AOAVM. (c) 2007 Elsevier Ltd. All rights reserved.