Nearly 40% of AIDS patients develop neurological complications during the course of their illness, and about 10% experience neurological symptoms as the initial manifestations of AIDS. The most common neurological complication (14% of AIDS patients) is human immunodeficiency virus (HIV) encepha-lopathy, but opportunistic viral and nonviral infections and neoplasms are also quite common; the most frequent among these are cryptococcal meningitis, toxoplasmosis, primary central nervous system (CNS) lymphoma, progressive multifocal leukoencephalopathy, and herpesvirus infections. Most of the nonviral infections and neoplasms are potentially treatable. Neurological syndromes include diffuse and regional encephalopathies, myelopathy, meningitis, intraaxial cranial neuropathies, and retinopathy. About 10% of AIDS patients develop a CNS mass lesion; the chief causes of these lesions are toxoplasmosis and primary CNS lymphoma. Since the clinical profiles of the various diseases overlap to a great extent, differential diagnosis requires a thorough workup, including magnetic resonance imaging or computed tomography brain scanning, examination of the cerebrospinal fluid, and, frequently, brain biopsy. Because AIDS patients have a high incidence of multiple intracranial pathologies, the diagnostic workup may have to be repeated to identify all of the diseases present.