The pathogenesis of late radiation necroses remains still unsolved in the problem of cerebral radiation injuries. The problem of primary site: vessels or medulla is a disputable one. In this case cerebral radiation nectosis developed in a 32-year-old woman 1.5 yrs. after the last course of radiotherapy for hypophyseal adenoma was over. The authors give an irradiation scheme and a detailed histological description of the foci of late radiation necrosis in the temporal lobe and in the substance of the Varolii pons which is not the zone of direct exposure. The histological picture described in the above foci is similar in many details to the process that takes place in experimental allergic encephalomyelites developing in sensitized animals. The time course of histological changes in the foci of radiation necroses shows that the process does not develop steeply but increases gradually affecting the adjoining zones of the medulla that cannot be accounted for by the direct involvement of the medullary tissue only during radiation. Thus it can be assumed that the process develops like an allergic reaction where the role of the allergen is played by the decomposition products of myelin that appear in the affected focus.