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°of flexion for more than 5% of the working time (RR 1.5, 95% Cl 1.0-2.1), for workers who worked with the trunk in a minimum of 30°of rotation for more than 10% of the working time (RR 1.3, 95% Cl 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% Cl 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.