For the geriatrician who commonly evaluates cognitive and psychiatric disorders in the elderly, the neurologic consequences of infection with the human immunodeficiency virus (HIV) are of particular importance. The most frequent neurologic disease is the AIDS dementia complex characterized by cognitive, behavioral, and motor changes, occurring in two-thirds of AIDS patients. The pathophysiology of central nervous system HIV infection has been advanced with important implications for both the diagnosis and the potential treatment of this devastating disease.