Brain MRI and/or CT were performed on 72 HIV-infected patients at various stages of the disease, and on 34 controls. The neuroradiological findings were related to duration of the infection, neurological symptoms, and cognitive abnormalities as well as to immunological findings in the CSF and blood. All types of brain atrophy were more severe and more frequent in HIV-infected subjects than in controls. Patients with neurological symptoms, those with advanced HIV infection, and patients with a duration of HIV infection of more than 4 years showed the most severe and most frequent neuroradiological abnormalities, including central and cortical atrophy, brain stem atrophy, and cerebellar atrophy. Subjects with cognitive defects exhibited more severe central atrophy than cognitively intact patients. However, slight brain atrophy and/or parenchymal lesions were found in 57% of cognitively intact HIV-seropositive individuals. Patients with brain atrophy and those with radiologically normal brain, both showed increased intrathecal synthesis of total IgG, and intrathecal HIV-antibody synthesis. However, a declined general immune response and a lowered CSF leukocyte count were seen predominantly in patients with brain atrophy. The results suggest that subcortical, neurologically "silent" areas of brain white matter are an early target of HIV infection.