The nervous system has been involved in the majority (at least 75%), of cases of acquired immune deficiency syndrome (AIDS) examined postmortem, but the pathogenetic mechanisms involved are not well understood. The predominant pathological process is opportunistic infection secondary to the decrease of T-helper (T4) cells and includes toxoplasmosis, encephalitis due to cytomegalovirus and progressive multifocal leucoencephalopathy. On the other hand, mycoses (mainly cryptococcosis) are relatively uncommon. Primary lymphomas are three times more common than secondary lymphoma spreading from other sites. Cerebral involvement by Kaposi sarcoma is metastatic, probably from primary foci in the lungs. Lesions due to the direct involvement of the nervous system by the human immune deficiency virus (HIV) include subacute encephalitis and vacuolar myelopathy. The former is reported with increasing frequency and is localized predominantly to the white matter in which multinucleated giant cells can be found. These are considered typical of AIDS and have been shown to contain HIV particles in their cytoplasm. AIDS lesions due to infectious agents do not always conform to the typical pattern of the uncomplicated disease and not uncommonly there is evidence of more than one infectious agent in the same area. Peripheral nervous system lesions in HIV infections, responsible for a variety of clinical symptoms, usually appear, in biopsy material, as nonspecific inflammatory in type. CMV inclusions and lymphomatous infiltrations of peripheral nerve have been reported in autopsy cases.