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Background: Aneurysmal subarachnoid haemorrhage (SAH) is a potentially devastating but treatable condition, the seriousness of which is not always immediately apparent. An invasive test: the lumbar puncture (LP), is considered a mandatory part of the diagnostic pathway according to guidelines but is frequently omitted in clinical practice. Aims: 1. To establish the extent to which current clinical practice diverges from recommendations. 2. To investigate the beliefs that inform clinician behaviour and establish if the demands required of a diagnostic test for SAH are best achieved by MRI or lumbar puncture. 3. To establish whether MRI has the potential to improve detection of spontaneous SAH compared to CT. Method: 1. The use of LP in cases of suspected SAH was audited at three major teaching hospitals in London. 2. A survey was conducted at major neuroscience centres in London to investigate clinicians’ experiences of SAH investigation and their expectations of diagnostic test performance. 3. A prospective imaging study compared the relative locational sensitivities of CT and MRI to detect subarachnoid haemorrhage in a group of clinically well patients. Results: LP was performed in a minority of patients undergoing CT as an investigation for suspected acute SAH (33%). Clinicians demonstrate wide-ranging opinions about the risks and benefits of SAH investigation strategies, with Emergency Medicine clinicians reporting significantly higher risk tolerances compared to Neuroscience clinicians. Blood-sensitive MRI sequences detected more regions of SAH compared to CT in a cohort of neurologically intact, treatment naïve, spontaneous SAH. Conclusion: Low adherence to the recommended diagnostic pathway exists in UK practice. This may reflect different risk-tolerances of clinicians and legitimate concerns about the utility of LP. Future guidelines should consider patient and doctor risk-tolerances when making recommendations. MRI has potential value as an alternative non-invasive test and could reduce time, cost and patient discomfort.