To find out if polymerase chain reaction could be used as a diagnostic tool for detecting neurosyphilis, we have applied the PCR for the detection of Treponema pallidum DNA in the cerebrospinal fluid (CSF) of syphilis patients. The results of the PCR of the CSF in 26 patients with at various stages of illness were compared with the results of other conventional tests used in the WHO criteria. T. pallidum was detected in the CSF of patients at all stages of syphilis, which indicates that they invade the central nervous system from the early stages of infection. However, the presence of T. pallidum in the CSF was not correlated with the results of other tests used in the WHO criteria, and its significance in the diagnosis of neurosyphilis should further be evaluated.