Background and Purpose The aim of our study was to investigate plasma and genetic risk factors for rupture of cerebral aneurysms.Methods In London, a case-control study was made of 56 consecutive patients admitted to a regional neurosurgical service for treatment of ruptured cerebral aneurysm and of 93 control subjects. A further 40 consecutive patients admitted in Arhus with ruptured cerebral aneurysm also were studied.Results The British case-control study showed that smoking was associated with an increased risk of ruptured cerebral aneurysm (odds ratio, 9.1; 95% confidence interval [CI], 3.4 to 23.8; P<.001 for a history of >10 pack years). After age and sex adjustment, factors associated with ruptured cerebral aneurysm included a cholesterol concentration in the highest tertile (^6.3 mmol/L; odds ratio, 10.2; 95% CI, 3.9 to 26.7; P<.001), an apolipoprotein B concentration in the highest tertile (a0.84 g/L; odds ratio, 6.4; 95% CI, 2.5 to 16.3; / > <.001), and concentrations of HDL cholesterol in the lowest tertile (<1.1 mmol/L; odds ratio, 3.6; 95% CI, 1.4 to 8.2; P<.01). History of hypertension was of less importance (odds ratio, 4.0; 95% CI, 1.41 to 11.7; /><.01). Smoking history