In this review of the acquired immunodeficiency syndrome (AIDS), the authors have evaluated a total of 352 homosexual patients with AIDS or generalized lymphadenopathy managed at the University of California, San Francisco (UCSF), between 1979 and 1984. Of an initial unselected group of 318 patients, 124 (39%) were neurologically symptomatic, and one-third already had their neurological complaints at the time of presentation. An additional 210 AIDS patients with neurological symptoms have been reported in the literature. Thus, a total of 366 neurologically symptomatic patients with AIDS or lymphadenopathy are reviewed. Central nervous system (CNS) complications, encountered in 315 patients, included the following viral syndromes: subacute encephalitis (54), atypical aseptic meningitis (21), herpes simplex encephalitis (nine), progressive multifocal leukoencephalopathy (six), viral myelitis (three), and varicella-zoster encephalitis (one). Non-viral infections were caused by Toxoplasma gondii (103), Cryptococcus neoformans (41), Candida albicans (six), Mycobacteria (six), Treponema pallidum (two), coccidioidomycosis (one), Mycobacterium tuberculosis (one), Aspergillus fumigatus (one), and Escherichia coli (one). Neoplasms included primary CNS lymphoma (15), systemic lymphoma with CNS involvement (12), and metastatic Kaposi's sarcoma (three). Cerebrovascular complications were seen in four patients with hemorrhage and five with infarction. Five patients in the UCSF series had multiple intracranial pathologies, including two cases of simultaneous Toxoplasma gondii infections and primary CNS lymphoma, two cases of coexistent Toxoplasma gondii and viral infections, and one case of combined Toxoplasma gondii and atypical mycobacterial infection. Cranial or peripheral nerve complications, seen in 51 patients, included cranial nerve syndromes secondary to chronic inflammatory polyneuropathy (five), lymphoma (five), and Bell's palsy (five). Peripheral nerve syndromes included chronic inflammatory polyneuropathy (12), distal symmetrical neuropathy (13), herpes zoster radiculitis (six), persistent myalgias (two), myopathy (two), and polymyositis (one). In light of the protean behavior of AIDS and the problems related to the clinical, radiological, and serological diagnosis of the unusual and varied associated nervous system diseases, patients with AIDS and neurological complaints require a rigorous and detailed evaluation. The authors' experience suggests that biopsy of all CNS space-occupying lesions should be performed for tissue diagnosis prior to the institution of other therapies.