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A population-based cross-sectional and 5-year prospective questionnaire study.To investigate self-reported physical workload as a risk factor for low back pain.Both physical and psychosocial workplace factors are considered risk factors for low back pain. However, today no consensus has been reached regarding the exact role of these factors in the genesis of low back pain.Questionnaire data were collected at baseline for 1397 (and after 5 years for 1163) men and women aged 31–50 years at baseline. Low back pain (“any low back pain within the past year,” “low back pain ≤ 30 days in total during the past year,” “low back pain > 30 days in total during the past year”) was analyzed in relation to physical workload (sedentary, light physical, and heavy physical work) using logistic regression and controlling for age, gender, and social group. The proportions of workers changing between the workload groups over the 5-year period were analyzed in relation to low back pain status.At baseline no statistically significant differences in low back pain outcomes were found for workers exposed to sedentary, light physical, or heavy physical work. This was true for all age, gender, and social groups. At follow-up there was a statistically significant dose–response association between any low back pain and longstanding low back pain within the past year and increasing physical workload at baseline also after controlling for age, gender, and social group. Subjects with heavy physical workload at baseline changed statistically significantly more often to sedentary work if they experienced low back pain for more than 30 days out of the past year.Having a sedentary job might have a protective or neutral effect in relation to low back pain, whereas having a heavy physical job constitutes a significant risk factor. Because of migration between exposure groups (the “healthy-worker” effect), longitudinal studies are necessary for investigating the associations between physical workload and low back pain.