HIV-infected patients are at markedly increased risk for neurological dysfunction, which may occur at any level of the neuraxis (see Table 1). The most common syndromes--AIDS dementia complex, vacuolar myelopathy, and possibly distal symmetric peripheral neuropathy--appear to be related to HIV infection within the nervous system, rather than due to the immunoincompetence caused by HIV. However, the mechanism(s) by which HIV causes these syndromes, e.g., infecting neurons or oligodendroglia directly, interfering with neurotrophic factors, effecting toxic monokine production, etc., is unknown. Early, albeit incomplete, success with azidothymidine is encouraging. Less commonly, neurological syndromes may be secondary to the immunoincompetence produced by HIV. Many different etiologies--most of which are treatable--have been encountered, but a few of these (cerebral toxoplasmosis, cryptococcal meningitis, primary CNS lymphoma, and progressive multifocal leukoencephalopathy) are responsible for most of the opportunistic complications. Marked differences in symptoms and signs between AIDS patients and immunologically normal patients may complicate recognition of some of these diseases (e.g., herpes simplex encephalitis). Finally, some HIV-associated syndromes, e.g., inflammatory demyelinating polyradiculoneuropathy and retinal microvasculopathy, are of unknown etiology.