OBJECTIVES: To establish if emergency medicine and neuroscience specialist consultants have different risk tolerances for investigation of suspected spontaneous subarachnoid haemorrhage (SAH), and to establish if their risk-benefit appraisals concur with current guidelines. SETTING: 4 major neuroscience centres in London. PARTICIPANTS: 58 consultants in emergency medicine and neuroscience specialities (neurology, neurosurgery and neuroradiology) participated in an anonymous survey. PRIMARY AND SECONDARY OUTCOME MEASURES: The primary outcome measure was the highest stated acceptable risk of missing SAH in the neurologically intact patient presenting with sudden onset headache. Secondary outcome measures included agreement with guideline recommendations, risk/benefit appraisal and required performance of diagnostic tests, including lumbar puncture. RESULTS: Emergency department clinicians accepted almost 3 times the risk of a missed SAH diagnosis compared with the neuroscience specialists (2.8% vs 1.1%; p=0.02), were more likely to accept a higher risk of missed diagnosis for the benefit of a non-invasive test (p=0.04) and were more likely to disagree with current published guidelines stipulating the need for LP in all CT-negative cases (p=0.001). CONCLUSIONS: Divergence from recognised procedures for SAH investigation is often criticised and attributed to a lack of knowledge of guidelines. This study indicates that divergence from guidelines may be explained by alternative risk-benefit appraisals made by doctors with their patients. Guideline recommendations may gain wider acceptance if they accommodate the requirements of the doctors and patients using them. Further study of clinical risk tolerance may help explain patterns of diagnostic test use and other variations in healthcare delivery.