In a cross-sectional study, the relationship of low back pain and sedentary work was examined among crane operators (n = 94), straddle-carrier drivers (n = 95), and a reference group of office workers (n = 86), aged 25 to 60 years. Information about history of low back pain, individual characteristics, and working conditions in past and present was obtained by a standardized interview. Assessment of postural load on the back was performed by observation of non-neutral postures of the trunk during normal work activities. Measurements of exposure to whole-body vibration in cranes and straddle-carriers were conducted. The 12-month prevalence of low back pain among crane operators was 50%; among straddle-carrier drivers, 44%; and among office workers, 34%. After adjustment for age and confounders, the odds ratio for newly developed cases of low back pain in the current job among crane operators was 3.29 (95% confidence interval, 1.52 to 7.12), and among straddle-carrier drivers 2.51 (95% confidence interval, 1.17 to 5.38). In both occupations the daily exposure to whole-body vibration was low, and therefore not considered an important risk factor for low back pain in this study. The observations showed that non-neutral postures of the trunk were frequently adopted among all workers. The results of this study suggest that sustained sedentary work in a forced non-neutral trunk posture is a risk factor for low back pain.