Neurologic disease occurs frequently in patients infected with the human immunodeficiency virus (HIV), and a large body of literature now exists detailing the various infections, neoplasms, and other conditions that can affect the central nervous system (CNS) or the peripheral nervous system in children and adults with AIDS, persistent generalized lymphadenopathy, or (in some cases) only serologic evidence of retroviral infection. Although certain opportunistic infections (toxoplasmosis, cryptococcosis, progressive multifocal leukoencephalopathy, and herpesviral infections) and CNS lymphomas often produce CNS disease in patients with AIDS, it is now clear that many cases of neurologic disease are caused by a group of disorders thought to be related to direct CNS infection by the AIDS retrovirus. Disease of the peripheral nervous system is also being increasingly recognized; some cases probably have an autoimmune basis.