Study Design. A 3-year prospective cohort study among workers of 34 companies in the Netherlands. Objectives. To investigate the relation between flexion and rotation of the trunk and lifting at work and the occurrence of low back pain. Summary of Background Data. Previous studies on work-related physical risk factors for low back pain either lacked quantification of the physical load or did not take confounding by individual and psychosocial factors into account. Methods. The study population consisted of 861 workers with no low back pain at baseline and complete data on the occurrence of low back pain during the 3-year follow-up period. Physical load at work was assessed by means of analyses of video-recordings. Information on other risk factors and the occurrence of low back pain was obtained by means of self-administered questionnaires. Results. An increased risk of low back pain was observed for workers who worked with the trunk in a minimum of 60[degrees] of flexion for more than 5% of the working time (RR 1.5, 95% CI 1.0-2.1), for workers who worked with the trunk in a minimum of 30[degrees] of rotation for more than 10% of the working time (RR 1.3, 95% CI 0.9-1.9), and for workers who lifted a load of at least 25 kg more than 15 times per working day (RR 1.6, 95% CI 1.1-2.3). Conclusions. Flexion and rotation of the trunk and lifting at work are moderate risk factors for low back pain, especially at greater levels of exposure. (C) 2000 Lippincott Williams & Wilkins, Inc.