ADC was first identified early in the AIDS epidemic as a common and novel CNS syndrome.(4,5) The three components of the term, AIDS dementia complex embody central features of the condition. AIDS emphasizes its morbidity and poor prognosis, particularly when its severity is at stage 2 or greater (see Table 1), a severity comparable to other clinical AIDS-defining complications of HIV-1 infection. Dementia designates the acquired and persistent cognitive decline with preserved alertness that usually dominates the clinical presentation and determines its principal disability. Complex emphasizes that this disease not only impairs the intellect, but also concomitantly alters motor performance and, at times, behavior. This involvement of the nervous system beyond cognition is evidence of a wider involvement of the CNS than occurs in some other types of dementia such as Alzheimer's disease. Additionally, myelopathy may be an important, indeed predominating, aspect of ADC, and organic psychosis may also be a feature in a subset of patients (see Rheumatologic and Musculoskeletal Manifestations of HIV). These manifestations are therefore also encompassed within this term. By contrast, neither neuropathy nor functional psychiatric disturbance are included in ADC.