Report of a case of polyradiculoneuritis with positivity of immunologic tests for syphilis in the CSF in a 36 years old man. Sensitive disturbances first appeared soon followed by ascending impairment of the motricity. One month later there occurred respiratory changes, diplopia, disphagia and involvement of the left facial nerve. Diplopia and respiratory disturbances disappeared one week later. Progressive remission of the remaining symptoms occurred later. Two months later, the neurologic examination showed only absence of the deep reflexes of the extremities, muscular hypotony and some degree of paresthesic sensations. Voluntary movements and muscle power were normal. Objetive disturbances of sensibility had disappeared. Nine months after the onset of the disease the neurologic examination was normal. CSF changes (table 1) were characterized by protein-cytologic dissociation and by the positivity of immunologic tests for syphilis. The protein pattern of the CSF was of the "mixed type" and the relative concentration of the albumin fraction was increased (paper electroforesis). Immunologic tests for syphilis were positive in blood serum also. Repeated spinal taps showed progressive reduction of the CSF protein concentration. The changes in the protein pattern tend to disappear with the decrease of the total protein concentration. Two months after the onset of the disease, the disturbances in the protein pattern were no longer present. The titers of the immunologic tests for syphilis in the CSF decreased rapidly. This fall accompanied the decrease in the total protein concentration. One month after the first CSF examination the immunologic tests for syphilis in the CSF were negative. Data concerning to the positivity of the immunologic tests for syphilis in the CSF were discussed. The changes were correlated to the alterations in the protein balance between blood and the CSF occuring in polyradiculoneurites rather than to neurosyphilis. Some data are in agreement with this idea. First, the decrease in the total amount of CSF proteins was accompanied by the decrease in the titer of antibodies. Also, the changes found in the protein pattern of the CSF were not similar to the findings in neurosyphilis. A marked increase in the y-globulin fraction is the main change in neurosyphilis. This was not found even when the total protein content of the CSF was moderately increased. The findings indicate that in the case reported the positivity of immunologic tests for syphilis observed in the CSF was secondary to the disturbances in the protein balance between blood and CSF. The passage of the imuno-proteins present in the blood to the CSF was a manifestation of the altered protein balance occurring in polyradiculoneuritis.