Lumbar puncture is a common procedure, and is part of the investigation to exclude two major diagnoses - meningitis and aneurysmal subarachnoid hemorrhage (SAH). There is always a concern, however that the procedure may worsen the condition of the patient by causing intracranial shift or coning. There is now good evidence for LP without CT after exclusion of specific criteria in both conditions, which will reduce the need for transport in rural areas. High risk criteria for aneurysmal SAH are itemized, and CSF investigations are clarified.