While dementia has been observed in approximately one-fourth of terminally ill patients with acquired immunodeficiency syndrome, it has been difficult to attribute this clinical disorder to a single neuropathological substrate. We used a simple and readily reproducible scale for estimating the burden of human immunodeficiency virus (HIV) in the central nervous system (i.e., severity of HIV encephalitis) and compared this to autopsy neurological summaries of dementia. Like others, we found that multinucleated giant cells were present in only half of the dementia patients. However, all of the dementia patients had severe HIV encephalitis as assessed by measurements of intra-central nervous system viral burden. Additional patients had severe HIV encephalitis without clinical histories of dementia. We interpret these latter findings as evidence that HIV encephalitis exists for a period of time before the clinical symptomatology develops. Comparison of presence or absence of concurrent cytomegalovirus encephalitis showed no association with dementia.