Being physically active is often suggested to be important in the prevention and management of low back pain. This simple view does not take into account that the relation between the level of activity and back pain may be a U-shaped curve - i.e. both inactivity and excessive activities (back-unhealthy activity) present an increased risk for back pain. We explored the U-shaped association between physical activity and chronic low back pain (3 months duration) by analyzing cross-sectional data from the Dutch population-based Musculoskeletal Complaints and Consequences Cohort study (DMC(3), 1998) of a sex-age stratified sample of 25 years and older (n=3364). Type of activity (daily routine, leisure time and sport activity), intensity of and time spent on these activities, and back exertion of sport activities were taken into account. Physical activity was not associated with chronic low back pain (CLBP) when studied by the dimension of activity, by the intensity or by the duration of physical activity. Only engaging in sport activity was associated with less CLBP (OR 0.78: 95% CI 0.66-0.93). The extremes of the total physical activity pattern were associated with CLBP. A moderate increased risk for CLBP was found for both participants with a sedentary lifestyle (OR 1.31: 95% CI 1.08-1.58) and for those being involved in physical strenuous activities (OR 1.22: 95% CI 1.00-1.49). This was especially true for women (sedentary: OR 1.44: 95% CI 1.10-1.83; physically active: OR 1.36: 95% CI 1.04-1.78). This study provides some evidence that the relation between physical activity and CLBP is U-shaped.