We report on 79 patients of different stages of human immune deficiency virus (HIV) infection according to the Walter-Reed staging classification (WR). Comparing the HIV antibody content per weight IgG in serum and cerebrospinal fluid (CSF), 54 patients (68%) showed higher antibody activity in CSF than in serum, indicating intrathecal antibody production and thus a local challenge with the virus. The percentage of patients with these antibodies in CSF increased from stage WR 1 (33%) to WR 5 (90%). It decreased again in WR 6 (68%). Twenty-one patients with intrathecally produced antibodies but without evidence for opportunistic or preexistent neuropsychiat-ric diseases were further analyzed. Even in stages WR 1 und 2 these patients showed distinct clinical signs. These consisted mostly in apathic personality change (n = 13), peripheral neuropathy (n = 8) or mild hemisyndrome (n = 9). Progression to severe dementia solely caused by HIV encephalitis seems to be possible. More often acceleration of the mental disorder indicates a synergistic action of other pathogens. Our study gives further evidence for very frequent, early and clinically active involvement of the nervous system by the HIV infection.