In recent years, incidence of low back pain (LBP) has been increasing. To clarify the relation between instability of the lumbar spine and LBP in heavy industry workers, radiographic findings were compared with a questionnaire on 2863 workers in a steel plant, as a preliminary study in 1986. In 214 males, 4-direction X-ray examinations of the lumbar vertebrae were obtained and the results were analyzed along with a questionnaire survey. The examined subjects were classified into three groups according to their complaints into : A ; LBP free, B ; LBP moderate, and C ; LBP severe. Degeneration chages of the facet joint, lumbar disc narrowing, angles between vertebrae, and lumbar lordosis were investigated on the X-ray film. In general, the higher the intensity of LBP, the more intervertebral angles at all levels of the lumbar spine and the more severe the degeneration. There was a statistically significant difference between the A and C group in the intervertebral angles at lower levels. Incidence of the lumbar disc narrowing differed between the two groups. The instabilitiy of lumbar spine was found in the C group. Low back pain in the heavy industry workers can be caused by anterior motion factors such as lumbar disc narrowing rather than posterior motion factors such as degeneration of the facet joint.