The central nervous system (CNS) has been examined at autopsy in 89 patients who died of the acquired immune deficiency syndrome (AIDS), including 14 patients who died primarily of neurologic complications of the disease. A total of 66 brains (74%) showed significant pathologic abnormalities, with opportunistic infections including cytomegalovirus (14) and cryptococcal (11) infections, progressive multifocal leukoencephalopathy (6), toxoplasmosis (6), and histoplasma microabscesses (1). Incidental Mycobacterium avium-intracellulare infection was found in 4 cases. Simultaneous CNS infection by more than one microorganism was encountered in 5 patients. Subacute (microglial nodule) encephalitis-related to cytomegalovirus infection or possibly brain infection by the causative agent of AIDS was present in 56 cases. Primary CNS lymphoma was noted in 3 patients. Secondary CNS deposits of lymphoma were found in 1 patient, and another patient had lymphomatoid granulomatosis. Vascular complications were not infrequently seen, and included infarcts secondary to vessel occlusion and disseminated intravascular coagulation in 4 patients and intracranial hemorrhage of variable severity in 13. White matter changes included vacuolar myelopathy (3 cases), central pontine myelinolysis (1 case), and foci of calcified, necrotizing leukoencephalopathy in pontocerebellar fibers of the basis pontis (2 cases). These findings highlight the variety of CNS complications in AIDS, some of which are not associated with clinical manifestations. Nevertheless, characterization of all lesions may be important in understanding the neurologic sequelae of AIDS.