To help determine the role of lumbar puncture in evaluation of patients with asymptomatic late syphilis, we reviewed results of cerebrospinal fluid (CSF) testing of 47 asymptomatic patients with syphilis. Syphilis was of unknown duration (n = 27) or known duration of greater than one year (n = 20), and all patients had reactive rapid plasma reagin (RPR) tests and reactive fluorescent treponemal antibody absorption tests. Thirty-two per cent of subjects had abnormal CSF findings; these included elevated protein in nine (19%) of 47 and pleocytosis in six (13%) of 47. In most cases, the etiology of these abnormalities was uncertain. Asymptomatic neurosyphilis, diagnosed on the basis of a reactive Venereal Disease Research Laboratory (VDRL; Atlanta, GA) test of CSF, was present in three (6%) of 47 patients. Of the eight patients with RPR titers of greater than or equal to 1:128, three (38%) had neurosyphilis. These results suggest that lumbar puncture is indicated for patients with asymptomatic late syphilis or syphilis of unknown duration.