The incidence of co-infection with Treponema pallidum and human immunodeficiency virus (HIV) is increasing in developing and developed countries. The neurological complications of both infections occasionally occur simultaneously during a clinical course. We herein report the case of an HIV carrier with syphilitic meningomyelitis and subclinical optic neuropathy. The patient presumably had latent syphilis and slowly developed longitudinally extensive transverse myelitis (LETM). A cerebrospinal fluid examination confirmed the diagnosis of active neurosyphilis based on an elevated T. pallidum hemagglutination assay index. A change in the patient's immune status, possibly due to HIV, might have converted the syphilis from latent to active, leading to LETM of the spinal cord.