In spite of the fact that animal experimentation has demonstrated the importance of the reticular formation of brain stem in the maintenance of consciousness some discrepancies still exist concerning the localization of this function in man. 40 patients suffering from intracranial tumours localized in brain stem diencephalon and basal ganglia have been reviewed by authors. From this study it could be observed that expanding lesions involving only brain stem did not show troubles of awareness which were also very seldom observed in tumours involving brain stem and neighbouring structures. A higher incidence of troubles of awareness was observed in patients with tumours involving diencephalon and basal ganglia. Troubles of consciousness seemed unrelated with increase of intracranial pressure because this increase was observed frequently in the first two groups of patients where troubles of consciousness were not present while troubles were observed in 73,4% of patients where intracranial pressure was not increased. A short review of troubles of consciousness in brain stem lesions of traumatic, vascular, and tumoral origin has been made by the authors and the discrepancies between neurophysiological data and clinical events is stressed in the light of the fact that hemispheric tumours more frequently than brain stem tumours cause troubles of consciousness. The possibility of a telencephalization of consciousness regulation in man is considered by authors.