In an attempt to elucidate the cause and mechanism of the dementia and other neurological disorders that can occur in HIV-1 infection, we have quantitatively assessed neuronal populations, by means of a stereological technique (the disector), in the frontal cortex of patients with HIV infection. Eleven of sixty-five brains in the Medical Research Council Central AIDS Brain Bank were selected for study. The selected patients died without opportunistic infection or neoplasm affecting the brain; they had HIV encephalitis or minimal changes. We compared their neuronal counts with those of eight control subjects (seven died of systemic illness, one of pontine haemorrhage which did not affect the cerebral hemispheres). The neuronal numerical density was significantly lower in the HIV group than in the control group (mean [SD] 307  vs 499  x 10(2) per mm3; p less than 0.001). This difference represents a loss of about 38%. There was no significant difference between the HIV subgroups, which suggests that neuronal loss occurs in cases of minor pathology as well as in HIV encephalitis. This finding contributes to the understanding of dementia in AIDS patients and has important implications for their future treatment.