CNS infections require prompt appropriate therapy, but do not usually require tissue biopsy for diagnosis. We performed a 5 year audit of CNS infections which required brain or spinal biopsy to determine or confirm a diagnosis of CNS infection. Sixteen cases were identified in which clinical, radiological or additional investigations including culture, serology or PCR for the suspected specific infective agents were not diagnostic. 6 (37.5%) were bacterial abscesses presenting as space-occupying intracerebral lesions with a differential diagnosis of neoplasm. There were 3 (18.7%) cases of toxoplasmosis and 2 (12.5%) cases of aspergillosis. There was one case (6.2%) of herpes simplex encephalitis, one cysticercosis and one progressive multifocal leucoencephalopathy, all biopsied as possible neoplasms. There were 2 (12.5%) cases of spinal tuberculosis, one multifocal, mimicking neurofibromatosis. This review highlights the usefulness of targeted biopsy in the rapid diagnosis of CNS infections. It also emphasizes the lack of specificity of 'negative' culture and serology in certain cases, especially in the setting of immune-compromise.