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A population-based, prospective cohort study.To determine whether overweight, obesity, or more generally an elevated body mass index (BMI) increase the probability of experiencing chronic low back pain (LBP) after an 11-year period, both among participants with and without LBP at baseline.Chronic LBP is a common disabling disorder in modern society. Cross-sectional studies suggest an association between an elevated BMI and LBP, but it is not clear whether this is a causal relationship.Data were obtained from the community-based HUNT 2 (1995–1997) and HUNT 3 (2006–2008) studies of an entire Norwegian county. Participants were 8733 men and 10,149 women, aged 30 to 69 years, who did not have chronic LBP at baseline, and 2669 men and 3899 women with LBP at baseline. After 11 years, both groups indicated whether they currently had chronic LBP, defined as pain persisting for at least 3 months continuously during the last year.A significant positive association was found between BMI and risk of LBP among persons without LBP at baseline. The odds ratio for BMI 30 or more versus BMI less than 25 was 1.34 (95% confidence interval [CI], 1.08–1.67) for men and 1.22 (95% CI, 1.03–1.46) for women, in analyses adjusted for age, education, work status, physical activity at work and in leisure time, smoking, blood pressure, and serum lipid levels. A significant positive association was also established between BMI and recurrence of LBP among women. LBP status at baseline had negligible influence on subsequent change in BMI.High values of BMI may predispose to chronic LBP 11 years later, both in individuals with and without LBP. The association between BMI and LBP is not explained by an effect of LBP on later change in BMI.