The paper presents 9 patients (5 women, 4 men, average age 58 years) with small infarctions located in either thalamus (T) or in the region of thalamofrontal ways. Such infarctions resulted in acute development of mental disorders which corresponded to dementia in 7 cases and to slight cognitive disorders in 2 cases. The complex of mental disorders resembled one in "Frontal Syndrome" and was characterised mainly by aspontaneity, adynamia, hypersomnia, disorientation, memory impairment, attention deficit, delay of all mental processes, lack of criticism and adequacy. Accompanying focal neurologic symptoms were slight in 7 patients, moderate or pronounced in 2 cases. In 5 patients the manifestation of mental disorders decreased gradually. Computer tomography revealed small infarctions in anterior or medial areas of T in 7 cases, and in thalamofrontal routes in 2 patients. Infarctions were located in dominant hemisphere of brain in 5 patients, in nondominant one--in 3 cases, in both hemispheres--in one patient. Location of all the foci corresponded to the structures through which the routes passed and connected T and reticular formation localized lower with frontal lobes of brain. It was suggested that separation of these routes resulted in cognitive disorders or dementia because of functional inactivation of the cortex of frontal lobes of brain.