In 2001, the incidence of primary and secondary syphilis increased in the United States for the first time in a decade. Increasing rates of early syphilis among men who have sex with men have been reported in many American cities, with similar outbreaks noted in Canada and Europe. In San Francisco, the increase has been particularly sharp and accompanied by an increase in the incidence of neurosyphilis. Early neurosyphilis develops within weeks to years of primary infection and primarily involves the meninges. Syndromes include syphilitic meningitis (often accompanied by cranial neuropathies), meningovascular syphilis (with associated ischemic stroke), or asymptomatic neurosyphilis. Late neurosyphilis occurs years to decades after exposure as cerebral or spinal gummatous disease or the classic parenchymal forms affecting the brain (general paresis or syphilitic encephalitis) or spinal cord and nerve roots (tabes dorsalis). Treponema pallidum, the causative agent, cannot be cultured in vitro, and microscopic techniques are laborious. Thus, diagnosis depends on serologic tests and cerebrospinal fluid (CSF) examination. The suboptimal sensitivity and specificity of these tests complicate diagnosis, particularly among patients coinfected with HIV. CSF examination should be performed to evaluate for neurosyphilis in all patients with positive serum syphilis serology and neurologic, ophthalmic, or tertiary disease, or in those who have failed therapy, and in HIVinfected patients with late latent syphilis or syphilis of unknown duration. Intravenous penicillin G is the recommended treatment for all forms of neurosyphilis and for syphilitic eye disease. An outpatient alternative, if adherence can be assured, is intramuscular benzathine penicillin with oral probenecid. Newer drugs that penetrate CSF, such as ceftriaxone or azithromycin, have not yet been adequately tested for neurosyphilis. Syphilis facilitates transmission of HIV (and vice versa), and thus all patients diagnosed with syphilis should be offered HIV testing.