BACKGROUNDS AND PURPOSE: The purpose of the study was to describe the characteristics, management, and outcomes of patients with confirmed aneurysmal subarachnoid hemorrhage and to compare outcomes across neurosurgical units (NSUs) in the UK and Ireland. METHODS: A cohort of patients admitted to NSUs with subarachnoid hemorrhage between September 14, 2001 and September 13, 2002 was studied longitudinally. Information was collected to characterize clinical condition on admission and treatment. Death or severe disability, defined by the Glasgow Outcome Score-Extended, was ascertained at 6 months. RESULTS: Data for 2397 patients with a confirmed aneurysm and no coexisting neurological pathology were collected by all 34 NSUs in the UK and Ireland. Aneurysm repair was attempted in 2198 (91.7%) patients (surgical clipping, 57.7%; endovascular coiling, 41.2%; other repair, 1.0%). Most patients (65.0%) were admitted to the NSU on the same day or the day after their hemorrhage; 32.0% of treated patients had the aneurysm repaired on the day of admission to the NSU (day 0), day 1 or day 2 and a further 39.3% by day 7. Glasgow Outcome Score-Extended at 6 months was obtained for 90.6% of patients (2172), of whom 38.5% had an unfavorable outcome. The median risk of an unfavorable outcome for all patients was 31% (5(th) and 95(th) percentiles, 12% and 83%), depending on prerepair prognostic factors. After adjustment for case-mix, the percentage of patients with an unfavorable outcome in each NSU did not differ significantly from the overall mean. CONCLUSIONS: In this study that collected representative data from the UK and Ireland, there was no evidence that the performance of any NSU differed from the average.