We add two cases of neoplastic angioendotheliosis to the previous 13 with central nervous system manifestations. This unusual neoplastic phenomenon is thought to represent a diffuse malignant proliferation of endothelial cells. A man aged 58 first had transient ischemic attacks followed by a subacute profound dementia with psychomotor retardation. Remission followed steroid therapy. Abulia recurred along with a gradually progressive paraplegia leading to death 16 months after onset of symptoms. Pathologic examination showed tumor masses in the basal ganglia and widespread patchy cerebral infarction. The spinal cord below the midthoracic level was destroyed. Microscopically small mononuclear tumor cells were widespread within and largely confined to the lumens of small cerebral blood vessels which showed varying degrees of occlusion and recanalization. Factor 8 antigen, a specific endothelial cell marker, was not present on the surface of the malignant cells. Another man aged 61 developed subacute dementia with frontal lobe signs, dysphasia and impairment of memory. Computerized tomography showed multiple infarcts. A brief respite followed steroid therapy but progressive deterioration led to death in 15 weeks. Frontal lobe biopsy revealed the same neoplastic vascular process. The hallmark of the illness is a progressive subacute lobar dementia. Transient spells may occur. Hemiparesis is unusual and seizures do not occur except terminally. Angiography is normal.