The clinico-pathological data and the topography of the lesions were determined in 13 cases of late radiation encephalopathy. In one case the arterial vascularisation was studied by the translucidation technique after filling of the blood vessels with a colloidal barium sulphate solution. The radiation lesions consisted of areas of focal necrosis and of diffuse demyelination and necrosis of the deep cerebral structures and the brain stem. Demyelination was predominantly present in cases of late appearance of the neurological symptoms while necrosis was found in cases with a short latency period. The cerebral cortex and the arcuate fibres were always the most preserved structures. The topography of the focal lesions in the cerebral hemispheres and in the brain stem corresponded well to the vascular supply areas of the deep perforating arteries, while the diffuse lesions always had a predominant distribution in the periventricular arterial end- and border-zones. These observations were also confirmed by a post mortem angiographic study. The present report argues once more for a vascular aetiology as cause of the late radiation encephalopathy.