The criteria for brain biopsy in patients with acquired immunodeficiency syndrome (AIDS) remain unclear and without universal acceptance. In order to shed more light on this issue, the authors reviewed the records of 25 AIDS patients with focal cerebral lesions who consecutively underwent stereotactic biopsy between November 1988 and October 1990. The most frequently occurring diagnoses were lymphoma (36%), progressive multifocal leukoencephalopathy (24%), and toxoplasmosis (8%). Patients whose central nervous system disease resulted in their initial presentation (approximately 40%) survived a median of 37 weeks, as opposed to 6 weeks for those who had previous AIDS-related infections. The proportion of biopsies of contrast-enhancing lesions that were diagnostic and thereby contributed to the patients' therapeutic management was 87.5%. On the other hand, only 67% of the biopsies of nonenhancing lesions were diagnostic, and none of these lesions were treatable. All of the lymphoma patients had had AIDS for some time and, despite a reasonable preoperative Karnofsky score and postoperative radiation therapy, their median survival was only 6 weeks; however, biopsy was critical to their therapeutic management. Early brain biopsy, rather than empiric antitoxoplasmosis therapy, appears indicated for aggressive therapy of contrast-enhancing lesions in patients who have had previous manifestations of AIDS. The role for biopsy of nonenhancing lesions is less clear, but it may provide prognostic information.