The authors analysed all cases of brain biopsy done in AIDS or HIV+patients at the Department of Clinical Neurosciences, Western General Hospital, Edinburgh, during the period 1989-92 to determine the clinical utility of the procedure. Thirteen lesions were biopsied revealing toxoplasmosis in five, lymphoma in four, progressive multifocal leucoencephalopathy in three, HIV encephalitis in two and encephalitis of unknown aetiology in one. One biopsy specimen contained abnormal tissue but was non-diagnostic. The median survival after biopsy was only 7 weeks, but preoperative immunological (median CD4 count 45) and clinical status of the patients were poor. Although the impact of tissue diagnosis on patient survival was minimal it did enable modification of drug therapy. The authors preliminary experience with biopsy in AIDS patients suggests that the diagnostic information obtained is of limited value in prolonging the lives of patients in poor immunological status, but may be useful in refining medical therapies for toxoplasmosis and progressive multifocal leucoencephalopathy. Whether biopsy should be performed earlier in the course of cerebral disease in an effort to prolong survival time remains unclear.