Cysticercosis of the nervous system in man produces the most varied neurologic and psychiatric syndromes, thus making clinical diagnosis difficult or impossible. Accurate diagnosis is attained only by means of the spinal fluid complement fixation test. With this procedure 168 cases have been diagnosed in a 12-year period. The clinical pictures more often seen were (1) generalized epilepsy, (2) intracranial hypertension, and (3) basal meningitis with mental changes. Attempts to make a diagnosis by other means (hematologic, X-ray examination) give unreliable results. The spinal fluid alterations are quite consistent pleocytosis with varied degree of eosinophilia, increased protein content with positive reaction for globulins, decreased sugar content, a negative Wassermann reaction and a positive complement fixation test for cysticercosis. The antigen employed is an alcoholic extract of desiccated and powderized cysticercus vesicles free of muscle or connective tissue. The technique is the same as that used for the Wassermann reaction but with a different antigen. The specificity of the reaction was checked against fluids with positive Wassermann, 30% gave positive cysticercosis reaction, therefore the specificity of the latter is conclusive only when the Wassermann reaction is negative. In long standing cases the cerebrospinal fluid (CSF) inflammatory changes tend to disappear but the positiveness of the reaction remains unchanged. Diagnosis of cysticercosis with the CSF examination has been verified in 18 cases at autopsy and in 40 at operation.