STUDY DESIGN: A cross-sectional population-based study. OBJECTIVE: To examine the association between body mass index and chronic low back pain, with adjustment for potential confounders. SUMMARY OF BACKGROUND DATA: Although many studies have investigated this association, it is still unclear whether there is a general relationship between body mass index and low back pain which applies to all populations. METHODS: This study is based on data collected in the HUNT 2 study in the county of Nord-Trøndelag in Norway between 1995 and 1997. Among a total of 92,936 persons eligible for participation, 30,102 men and 33,866 women gave information on body mass index and indicated whether they suffered from chronic low back pain (69% participation rate). A total of 6293 men (20.9%) and 8923 women (26.3%) experienced chronic low back pain. Relations were assessed by logistic regression of low back pain with respect to body mass index and other variables. RESULTS: In both sexes, a high body mass index was significantly associated with an increased prevalence of low back pain. In men the estimated OR per 5 kg/m increase in body mass index was 1.07 (95% CI: 1.03-1.12) and in women 1.17 (95% CI: 1.14-1.21), after adjustment for age, with a significantly stronger association in women. Additional adjustment for education, smoking status, leisure time physical activity, employment status, and activity at work hardly affected these associations. No interactions were found with most other factors. CONCLUSION: This large population-based study indicates that obesity is associated with a high prevalence of low back pain. Further studies are needed to determine if the association is causal.