We compared cerebrospinal fluid (CSF) opening pressure measurements in the lumbar subarachnoid space between the flexed position (F-OP) and relaxed position (R-OP) in recumbent patients. We devised an equation for using F-OP to determine the existence of raised intracranial pressure (ICP). Patients (n = 83) underwent lumbar puncture while in the flexed lateral decubitus position and then were moved to the relaxed position. F-OP and R-OP were measured with a water manometer. R-OP > 180 mmH2O plus relevant clinical signs were taken as indicators of raised intracranial pressure. Mean pressures for F-OP and R-OP were 178.54 and 160.52 mmH2O respectively, p <0.001. When F-OP > 180, raised ICP could be significantly over diagnosed. The authors recommend an equation [R-OP(calculated, mmH2O) = 0.885 x F-OP(measured, mmH2O)] or using 200 mmH2O as the threshold for increased ICP with flexed posture.