In patients with abnormal neuropsychiatric symptoms and a reactive fluorescent treponemal antibody absoption (FTA-ABS) test in the cerebrospinal fluid (CSF) or serum, a normal CSF cell count and total protein concentration does not exlude late syphilis involving the central nervous system. In these patients, the presence of plasma cells in the CSF cytogram, increased concentration of CSF immunoglobulin G (IgG), immunoelectrophoretic abnormalities in the precipitates of the IgG and of the Fab fragments of IgG in the CSF immunoelectropherogram, and an increased serum level of immunoglobulin M (IgM) suggest an active, potentially treatable neurosyphilis.