OBJECTIVERisk factors for growth of unruptured intracranial aneurysms (UIAs) during a lifelong follow-up in relation to subsequent rupture are unknown. The author’s aim in this study was to investigate whether risk factors for UIA growth are different for those that lead to rupture than for those that do not.METHODSThe series consists of 87 patients with 111 UIAs diagnosed before 1979, when UIAs were not treated. A total follow-up time of the patients was 2648 person-years for all-cause death and 2182 years when patients were monitored until the first rupture, death due to unrelated causes, or the last contact (annual incidence of aneurysm rupture, 1.2%). The follow-up time between aneurysm measurements was 1669 person-years. Risk factors for UIA growth were analyzed in relation to subsequent rupture.RESULTSThe median follow-up time between aneurysm measurements was 21.7 years (range 1.2–51.0 years). In 40 of the 87 patients (46%), the UIAs increased in size ≥ 1 mm, and in 31 patients (36%) ≥ 3 mm. All ruptured aneurysms in 27 patients grew during the follow-up of 324 person-years (mean growth rates 6.1 mm, 0.92 mm/year, and 37%/year), while growth without rupture occurred in 13 patients during 302 follow-up years (3.9 mm, 0.18 mm/year, and 4%/year) and no growth occurred in 47 patients during 1043 follow-up years. None of the 60 patients without aneurysm rupture experienced one during the subsequent 639 follow-up years after the last aneurysm measurement. Independent risk factors for UIA growth (≥ 1 mm) in all patients were female sex (adjusted OR 3.08, 95% CI 1.04–9.13) and smoking throughout the follow-up time (adjusted OR 3.16, 95% CI 1.10–9.10), while only smoking (adjusted OR 4.36, 95% CI 1.27–14.99) was associated with growth resulting in aneurysm rupture. Smoking was the only independent risk factor for UIA growth ≥ 3 mm resulting in aneurysm rupture (adjusted OR 4.03, 95% CI 1.08–15.07). Cigarette smoking at baseline predicted subsequent UIA growth, while smoking at the end of the follow-up was associated with growth resulting in aneurysm rupture.CONCLUSIONSCigarette smoking is an important risk factor for UIA growth, particularly for growth resulting in rupture. Cessation of smoking may reduce the risk of devastating aneurysm growth.