Between 1982 and 1988, 174 brains were systematically collected from consecutive, autopsied AIDS patients in a Parisian general hospital without neurology and psychiatry departments. The data obtained under these conditions provide reliable information on the frequency of central nervous system (CNS) involvement in a non-selected population of AIDS patients, most of whom were homosexuals (75.9%). One or several lesions were observed in 148 cases (85%). HIV encephalitis and/or leucoencephalopathy with multinucleated giant cells was found in 33 cases (18.9%). Opportunistic infections were identified in 91 patients (52.3%): toxoplasmosis (65 cases; 37.3%), cytomegalovirus encephalitis (25 cases; 14.3%), cryptococcosis (9 cases; 5.8%), progressive multifocal leukoencephalitis (5 cases; 2.8%), candidosis (1 case) and tuberculosis (1 case). Neoplasias were observed in 23 patients: primary (16 cases; 17.9%) or secondary malignant non Hodgkin's large B-cell lymphoma (3 cas; 1.1%), Kaposi's sarcoma (1 case) and glioma (3 cases; 1.1%). Non-specific lesions (vasculitic, hemorrhagic, metabolic and especially microglial nodules) were common. During the 6 years of study, the rate of CNS involvement was constant. The number of toxoplasmosis cases per year was stable, however, evolutive forms were more prevalent between 1982 and 1986, whereas treated inactive lesions were seen most frequently thereafter. The opportunistic complications were often associated and it should be noted that HIV encephalitis was associated with one of several such infections in 85% of the patients. This high rate of association suggests that these opportunistic infections may play a role in the pathogenesis of HIV encephalitis.