Several studies concerning risk factors for SAH and for subsequent rupture of an unruptured aneurysm have been published, but not risk factor studies for formation and growth rate of aneurysms. Because less than half of all aneurysms ever rupture, it is essential to know risk factors separately both for aneurysm formation and for its growth. Before 1979, unruptured aneurysms were not operated on in Helsinki. Recently, the results of risk factors for rupture of unruptured aneurysms of 142 patients (131 with a prior SAH) have been published. 89 were followed with conventional and/or 3D CT angiography, or at autopsy to define risk factors for aneurysm formation and growth. During 2575 person-years, 33 of the 142 patients (23%) suffered SAH, resulting in an annual incidence of 1.3% (95% CI, 0.9-1.7%). The cumulative rate of bleeding was 10.5% (95% CI, 5.3-15.8%) at 10 years, and 30.3% (21.1-39.6%) at 30 years. Independent risk factors for rupture were cigarette smoking (time-dependent relative risk, 3.04; 95% CI, 1.21-7.66), and size of aneurysm (1.14 per mm; 1.01-1.30) after adjustment for age, aneurysm group, and hypertension. In addition, current cigarette smoking at end of follow-up (age-adjusted odds ratio, 3.92; 95% CI, 1.29-11.93) and female gender 3.36 (1.11-10.22) were the only independent risk factors for aneurysm growth of > or = 1 mm but only current smoking (3.48, 1.14-10.64) was a risk factor for growth of > or = 3 mm. Probability of de novo aneurysm formation was 0.84% per year (95% CI, 0.47-1.37%). Female gender (adjusted odds ratio, 4.73; 95% CI, 1.16-19.38) and current smoking (4.07, 1.09-15.15) were the only significant (p < 0.05) independent risk factors for de novo aneurysm formation. Cessation of smoking is very important for these patients. It is recommended that unruptured aneurysms be operated on irrespective of their size and of patients' smoking status, in people aged < 50 to 60 years.