This paper reports 8 cases of Polycythemia Vera registered in the "Hospital das Clínicas da Faculdade de Medicina da Universidade de São Paulo" since 1944 till June-1955, on a total of 147,749 patients. In all cases there were symptoms and/or signs imputable to lesions of the nervous system. In 5 of the patients the symptoms were of a subjective kind (headache, dizzyness and asthenia); in only one of these 5 cases there were anamnestic references to a transient left hemiparesis. In 3 of them the authors registered a marked retinal venous congestion; in another, the "fundus oculi" was normal; in the last case no ophtalmoscopic examination was made. In the 3 remaining patients, besides identical subjective symptoms, there were neurologic focal signs. In the first one (case 6) there were signs of occlusion of the medium left cerebral artery; in the second (case 7) the clinical picture consisted of a sensitive-motor paraplegia; in the last one (case 8) the Wallemberg syndrome was manifest, indicating the occlusion of the left posterior inferior cerebelar artery. Referring the pathogenic hypothesis which seek connections between the Polycythemia Vera and the diencephalic lesions or the existence of intracranial hemangioblastomas, the authors think that the neurologic phenomena occurring with Polycythemia Vera must be explained by various factors, especially the increased blood viscosity which increases the cerebro-vascular resistance, the lessening of the flow and speed of the blood in the brain, and the arteriosclerosis, the development of which seems to be favoured by polycythemia conditions. The authors cast forth the hypothesis that the paraplegia found in one of their patients (case 7) must be due to an intraraquidian angioma similar to the vascular tumors which are frequently associated with Polycythemia Vera.