||OBJECTIVES: Life-style factors have been associated with the risk for aneurysmal subarachnoid hemorrhage (aSAH), but it is not clear whether body mass index (BMI) and serum lipids are associated with risk. We prospectively assessed these associations in two large population studies. METHODS: A total of 65,526 participants in the Nord-Trondelag Health Study (1995-1997) and 26,882 participants in the Tromso Study (1994-1995) were included. Studies included measurements of body weight and height, serum lipids, and self-administered questionnaires. Participants who experienced aSAH were identified, and hazard ratios (HRs) were estimated using Cox regression analysis. RESULTS: During 11 years of follow-up, aSAH was diagnosed in 122 participants. Overweight (BMI 25-29.9) was negatively associated with the risk of aSAH (HR 0.7, 95% CI 0.4-1.0). There was no over all association of total serum cholesterol, HDL cholesterol, or triglycerides with the risk of aSAH, but in participants younger than 50 years, HDL cholesterol was inversely associated with the risk (HR per standard deviation increase 0.6, 95% CI 0.4-0.9). CONCLUSIONS: Overweight may be associated with reduced risk of aSAH, but there was no over all association of total serum cholesterol, HDL cholesterol, or triglycerides with the risk of aSAH in this prospective study.