Between 3.6 and 6% of the population harbour an unruptured intracranial aneurysm. Risk of rupture is related to aneurysm site and size and whether or not the patient has already had a subarachnoid haemorrhage (SAH) from another aneurysm. In ISUIA 2, the rupture rate for anterior circulation aneurysms<7mm was 0% per year in patients with no prior SAH, and 0.3% per year in patients with previous SAH; 7-12mm aneurysms, 0.5% per year (both groups); 13-24mm aneurysms, 3% per year; and giant aneurysms 8% per year. Rupture rate for posterior circulation aneurysms is higher at all sizes:<7mm was 0.5% per year in subjects with no prior SAH, 0.7% in those with prior SAH; 7-12mm, 3% per year; 13-24mm, 3.7% per year; and giant aneurysms, 10% per year. Non-invasive tests like contrast enhanced magnetic resonance angiography (MRA) and multislice computed tomographic angiography (CTA) are alternatives to intra-arterial digital subtraction angiography (IADSA) to detect aneurysms. Although these are promising techniques, the quality of data testing their accuracy remains limited and single slice CTA and time-of-flight MRA are poorer at detecting aneurysms<5mm diameter, which account for up to 1/3 of unruptured aneurysms. For ruptured aneurysms, the only large scale randomised controlled trial comparing surgical and endovascular treatment (ISAT) by coiling, resulted in an absolute 8.8% reduction (updated figure as of June 2003 for 1888 patients) in death or dependency at 1 year compared with surgical clipping. For unruptured aneurysms, the best available data so far comparing coiling and clipping is from the prospective (but non-randomised) arm of ISUIA. Elective surgical clipping had combined morbidity and mortality at 1 year of 12.2% versus 9.5% for coiling, although the groups were not matched with more high risk patients in the endovascular treatment cohort. Nevertheless these data are encouraging for future randomised trials of elective coiling versus clipping for asymptomatic aneurysms, in particular as the unproven long-term durability of coiling treatment and the fact that complete aneurysm occlusion is not always achieved remain obstacles to its wider use in unruptured aneurysms. There is an increased risk of SAH in relatives of patients with SAH (highest in those with two or more first degree relatives affected), but most SAH is sporadic and therefore the balance of available evidence indicates that mass screening for aneurysms is not cost effective. There may be a limited role for investigation of high-risk subgroups and ideally such screening should be tested in a randomised trial. The avoidance and active management of vascular risk factors should also be part of the management of at risk subjects.