Since its appearance in 1981, the acquired immunodeficiency syndrome (AIDS) has had a staggering medical, social, and economic impact. More than 50,000 cases have been reported in the United States resulting in over 30,000 deaths. By 1991, it is estimated that there will be 270,000 cases of AIDS with 1.5 million carriers in the United States, costing an estimated 8.5 billion dollars in personal medical care costs and 55.6 billion dollars in indirect costs. Less appreciated is the fact that 10 per cent of cases of AIDS occur in persons greater than 50 years of age with 25 per cent of these in persons greater than 60 and 4 per cent in persons over 70 years of age. If the present distribution holds, there would be an estimated 27,000 cases of AIDS in persons greater than 50, with 1100 cases over 70 years of age within the next 3 years. 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 disorder is subacute encephalitis, recently termed the AIDS dementia complex. This syndrome is characterized by a triad of dementia, behavioral difficulties, and motor system dysfunction, occurring in up to two-thirds of AIDS patients, presenting as the initial manifestation of disease in 25 per cent of them. In one recent autopsy series, 90 per cent of patients with AIDS related complex (ARC) or AIDS had pathologic findings of subacute encephalitis. In addition, one-half of asymptomatic HIV carriers have been shown to have cognitive deficits on neuropsychometric testing. Central nervous system opportunistic infection and neoplasia are also important considerations in the evaluation of patients with neurologic dysfunction.