1. This thesis consists of the systematic clinical analysis of 26 personally observed cases of cerebral syphilis, in seven of which the diagnosis was confirmed by autopsy.2. Cerebral syphilis plays an important part in the production of mental disease, and should occupy a more prominent place among the organic psychoses than it heretofore has done.3. The spirochaæte pallida has for long been surmised to be the causal organism, but it was not until 1910 that Strasmann first demonstrated its presence in the central nervous system of an adult with acquired syphilis; the second case is reported in this thesis.Trauma, alcoholism, and physical and mental strain are important contributory factors.Re-infection with syphilis is quite possible provided the initial infection has been thoroughly cured.4. Anatomically, three main types of cerebral syphilis are differentiated; viz., meningitis, endarteritis, and gumma. Clinically, this differentiation is seldom possible, and is without practical value, as the treatment is the same in all irrespective of the type.5. The majority of cases of cerebral syphilis develop within the first three years after primary infection, and rarely more than ten years after infection; this is in striking contrast to cases of general paralysis and locomotor ataxia, which almost invariably develop at a period more than ten years after infection.6. In regard to the physical signs, the Argyll Robertson phenomenon is the one on which most weight should be laid in differential diagnosis, as it is rarely present in cases of cerebral syphilis. Other important features are: (a) an acute onset with headache, dizziness, and vomiting; (b) cranial nerve palsies; (c) convulsions without loss of consciousness, but usually followed by permanent focal symptoms; (d) intactness of speech and writing; (e) absence of facial tremor.7. Cerebral syphilis not infrequently causes pseudo-bulbar paralysis, and six cases of this affection have been here reported.8. The mental symptoms of cerebral syphilis are of the nature of those seen in acute organic reactions, and consist of confusion, delirium, amnesia, hallucinations, retention defect, and a poor memory for recent events; in addition there is relatively little disintegration of the personality.9. The Wassermann reaction must be considered in relation with the clinical picture in each individual case; when the Wassermann reaction with the cerebro-spinal fluid is negative, the diagnosis of cerebral syphilis is indicated.10. It is frankly admitted that there is no pathognomonic sign for cerebral syphilis; but if the nature and character of the onset and the above-mentioned physical and mental symptoms and signs are correlated, a disease entity is formed which has every right to be considered characteristic.11. Anomalous features, among which may be mentioned euphoria and grandiose ideas, and confabulatory states, are more common in cerebral syphilis than is generally recognized; special attention must be paid to the setting in which these features occur, because when occurring in a setting of confusion they mean practically nothing.12. Recent statistics confirm one in the opinion that the prognosis of cerebral syphilis, as compared with other organic affections of the nervous system, is relatively good; the most favorable cases are those which develop soon after the primary infection, and those of a meningitic or gummatous type.13. Mercury, no matter in what form administered, is an exceedingly valuable drug in the treatment of syphilis, provided that it is given in a systematic way. The best results are, however, probably obtained by combining mercurial and salvarsan treatment. Potassium iodide acts simply as an eliminative agent, and has no specific action on the spirochæte pallida.The only safe treatment is prophylaxis.