The case reported here is that of a woman, without any significant pathological antecedent. At age 54, she developed signs of thalamic dementia and died 5 years later. The prominent symptoms were massive anterograde amnesia, apathia, apragmatism, ataxia and a frontal syndrome. She never showed aphasia, apraxia, agnosia or disorders of ocular motility. The C. T. Scan showed lacunar low densities in the mesencephalon and both thalami, mainly on the left side, as well as hydrocephalus caused by a stenosis of the aqueduct as shown by other neuroradiological procedures. The neuropathological examination showed space-occupying lacunae, bulging in the third ventricle, squeezing the aqueduct and protruding into the fourth ventricle. These lacunae were situated in the territory of the paramedian mesencephalo-thalamic arterial pedicle. They were perivascular spaces distensions, probably caused by disorder of the permeability of the arterial wall. This hypothesis is supported by the presence of severe lesions of segmental necrotizing angeitis on a paramedian mesencephalic artery. The etiology of this angeitis is unknown. As far as we know, such neuropathological lesions have never been reported previously. Therefore the pathology and etiopathogeny of cerebral lacunae should be reconsidered.