Study Objectives: To determine the prevalence of Treponema pallidum in cerebrospinal fluid (CSF) of patients with syphilis, to determine the effect of concurrent HIV infection on central nervous system involvement by T. pallidum, and to examine the efficacy of conventional therapy for asymptomatic neurologic involvement. Patients: Fifty-eight patients with untreated syphilis who consented to lumbar puncture, representing approximately 10% of new cases of syphilis during the study period. Interventions: Lumbar puncture was done on all patients. Rabbit inoculation was used to test cerebrospinal fluid for viable T. pallidum. Patients with normal fluid received recommended benzathine penicillin therapy according to the stage of syphilis; patients with CSF abnormalities were offered 10-day therapy for neurosyphilis. Results: Treponema pallidum was isolated from the CSF of 12 (30%) of 40 patients (95% CI, 17 to 46) with untreated primary and secondary syphilis; isolation of T. pallidum was significantly associated (P = 0.008) with the presence of two or more abnormal laboratory variables (among leukocyte count, protein concentration, and CSF-Venereal Disease Research Laboratory [VDRL] test). Two (67%) of 3 early latent (CI, 13 to 100) and 3 (20%) of 15 late latent syphilis patients (CI, 5 to 47) also had reactive CSF-VDRL tests and elevated cell and protein levels, although T. pallidum was not isolated. Concurrent infection with the human immunodeficiency virus (HIV) was not associated with isolation of T. pallidum, increased number of CSF abnormalities, or reactive CSF serologic tests for syphilis, although CSF pleocytosis was commoner in subjects infected with HIV. Treatment with conventional benzathine penicillin G (2.4 mIU) failed to cure 3 of 4 patients with secondary syphilis from whom T. pallidum was isolated before therapy; all 3 patients in whom treatment failed were HIV seropositive when treated or seroconverted during follow-up. Conclusions: Central nervous system invasion by T. pallidum is common in early syphilis, and is apparently independent of HIV infection. Examination of the CSF may be beneficial in patients with early syphilis, and therapy should be guided by knowledge of central nervous system involvement. Conventional benzathine penicillin G therapy may have reduced efficacy in patients with early syphilis who are also infected with HIV.