People who work at construction sites have one of the highest incidences of low back pain (LBP). The purpose of this study is to evaluate the status of LBP, and to investigate the risk factors of LBP in construction workers. The subjects were 33, 530 workers who were sent our questionnaires in September and October 1996. Of the final respondents, 29.3% had LBP. The results of multi-regression analysis revealed that risk factors having significant relationships with the onset of LBP were 1) stress due to personal relations at work, 2) postures during work, and 3) unstable body balance on scaffoldings. In addition, two factors were found to be important in decreasing the incidence of LBP: 1) having sufficient space for taking a rest and 2) using pre-work exercises. These findings have led us to the conclusion that not only providing good physical working environments, but also giving instructive and psychological care are important in preventing the onset of LBP in construction workers.