The authors present the cases of nine patients with acquired immunodeficiency syndrome (AIDS) and intracerebral mass lesions, who were evaluated at the University of California, San Francisco, between April, 1979, and July, 1983. Eight patients were confirmed homosexual males, and none was Haitian . Their average age was 38.8 years. Tissue diagnosis was made in all patients from brain biopsy or autopsy material. Three patients initially presented for evaluation of their neurological deficits, while the other six already carried the diagnosis of AIDS at admission. Seven patients presented with multiple intracranial lesions and two had polymicrobial infection. In this series, three patients had Toxoplasma gondii brain abscesses, two had primary lymphoma, two had metastatic Kaposi's sarcoma of the central nervous system (CNS), two had focal cytomegalovirus encephalitis and one each had cryptococcal and Candida albicans brain abscesses. The clinical presentation, radiological evaluation, and serodiagnostic study of these patients were not helpful in establishing the nature of the CNS lesions. Brain biopsy is considered by the authors to be critical for the evaluation and appropriate treatment of mass lesions in patients with AIDS.