Subarachnoid haemorrhage (SAH) is a potentially life-threatening cause of acute severe headache. This service evaluation aimed to investigate the prevalence of SAH in patients referred to acute medicine with acute severe headache who underwent a lumbar puncture (LP) at a tertiary teaching hospital, over a four-year period. Of the 677 samples analysed for CSF bilirubin, 619 were negative (91.5%), 48 indeterminate (7%), and 10 classified as positive (1.5%). Only one patient (0.1%) was diagnosed with an aneurysmal SAH and underwent intervention. Therefore, the diagnostic yield of an LP to exclude SAH in this setting is extremely low. This service evaluation challenges the dogma that LP is necessary in all patients with acute severe headache.