We report a prospective investigation of a bedside test to evaluate the role and safety of lumbar puncture in raised intracranial pressure in patients with subarachnoid haemorrhage. Fourteen patients who underwent aneurysm clipping following subarachnoid haemorrhage were studied. All patients had intraventricular drains and needed high volume cerebrospinal fluid (CSF) drainage to maintain the normal intracranial pressure. A lumbar puncture was performed in these patients and the simultaneous opening lumbar and ventricular pressures noted, CSF was then drained via the lumbar puncture, and the simultaneous closing lumbar and ventricular pressures noted. In all patients, the opening lumbar pressure was close to the ventricular pressure. In 13 of 14 patients, CSF drainage resulted in an equivalent and simultaneous fall of ventricular pressure. We concluded that simultaneous measurement of lumbar and ventricular CSF pressure before and after lumbar CSF drainage allows identification of candidates with differential cranial and lumbar pressures and therefore indicates safety or risk of lumbar CSF drainage.