The variability of exposure to postural load on the back was studied in five occupational groups. A random sample of workers in each group was observed for two periods of 30 min during a shift, their posture being classified every 20 s. The estimated percentage of time spent in trunk flexion and rotation formed the principal measures of exposure. The partitioning of the total variability of exposure showed that occupational group status was the principal source of variance. The between-group variance accounted for 47 and 72% of the total variability of exposure to trunk flexion and rotation, respectively. The corresponding percentages were 29 and 16% for the within-worker variance of trunk flexion and rotation and 24 and 12% for the between-worker variance. This type of analysis of the sources of exposure variability may help to establish appropriate measurement strategies for exposure to postural load on the back in epidemiologic studies on low-back pain.