The central nervous system was examined in 40 AIDS patients who died between August 1982 and 1987. The cases included two children born to intravenous drug abusers and 38 male adults. The brains of eight patients who had no clinical or radiological evidence of central nervous system involvement showed non-specific changes which included microglial nodules, perivascular mononuclear cuffs, mineralization of blood vessels and granular ependymitis. In 32 brains from patients with neurological symptoms, toxoplasmosis was the most frequent finding (19 cases) manifested by multifocal, necrotic lesions or a diffuse pseudo-encephalitic process. Other opportunistic infections included cytomegalovirus (eight cases), progressive multifocal leucoencephalopathy (two cases), cryptococcosis (one case), aspergillosis (one case), multiple bacterial microabscesses (one case) and Mycobacterium avium intracellulare (one case). Two patients had cerebral lymphoma. Subacute encephalitis with white matter lesions and multinucleated giant cells characteristic of HIV infection was present in 15 cases. Various combinations of all these infections were encountered in the same brain, sometimes in the same area and, occasionally, in the same cell.