The brains of 65 individuals with antibodies to human immunodeficiency virus type 1 (HIV-1), 20 HIV-1 seronegative homosexual men, and 75 heterosexual controls were examined by a quantitative magnetic resonance imaging technique. A white matter aberration was detected most frequently in patients with AIDS-related complex (ARC) or AIDS, but also in asymptomatic HIV-1 seropositive persons and in HIV-1 seronegative homosexual men, of whom two of three tested were reactive for HIV-1 DNA by polymerase chain reaction. The aberration was not found in the control group. Brain atrophy was mainly confined to patients with ARC or AIDS. The brain lesions correlated with the presence of HIV-1 in cerebrospinal fluid and with elevated levels of beta 2-microglobulin and neopterin. The most pronounced brain aberrations were in patients with AIDS-dementia complex. These findings indicate that brain aberrations may occur in persons in the early stages of HIV-1 infection, although to no greater extent than in HIV-1 seronegative homosexual men. The occurrence of pronounced brain lesions seems to be associated with the presence of an advanced immunodeficiency.