STUDY DESIGN: Community-based, cohort study. OBJECTIVE: Our aim was to determine the course of back pain in middle-aged women over a 9-year period, and assess whether obesity and physical inactivity are associated with more frequent back pain. SUMMARY OF BACKGROUND DATA: Back pain is the leading cause of disability worldwide. With minimal effective therapies and rising financial burden, identifying modifiable risk factors remains a key priority. METHODS: The Australian Longitudinal Study on Women's Health is a cohort study of community-based, middle-aged women who completed questionnaires every 3 years between 2004 and 2013. Approximately, 10,530 women completed the survey in 2004 (mean age 55.5 yrs), and 9020 completed follow-up 9 years later. Self-reported data on back pain in the last 12 months and other sociodemographic factors were collected at all four surveys. 'Frequent back pain' was defined as back pain reported at ≥ three surveys. RESULTS: Back pain was common and persistent, with 48% having back pain in ≥ three out of four surveys. Baseline obesity (relative risk [RR] 1.18, 95% confidence interval [CI] 1.12-1.25), lack of vigorous physical activity (RR 1.17, 95% CI 1.10-1.25), depressive symptoms (RR 1.40, 95% CI 1.33-1.47), and low-education status (RR 1.17, 95% CI 1.12-1.24), were independently associated with an increased risk of frequent back pain (all P < 0.001). Overall, 28% of the risk of frequent back pain could be attributed to these factors, equating to one extra case of frequent back pain for every five women with depressive symptoms, for every 11 obese women, for every 12 women with low-education status, and for every 13 women who do not do vigorous physical activity, at baseline. CONCLUSION: Obesity, depressive symptoms, low-education status, and lack of vigorous physical activity are associated with higher risk of frequent back pain over the following 9 years among women in their mid-50 s. Targeting these risk factors may lessen the burden of back pain.2.