B-scan ultrasonic measurements of lumbar spinal canal diameter were examined as predictors of industrial back pain complaints and extended work loss. Baseline data were collected on 3,020 Washington State aircraft manufacturing workers, and over a mean 3.7-year follow-up period 352 subjects reported industrial back pain complaints. Mean canal measurements of subjects with industrial back pain complaints were smaller at all spinal levels than in subjects without complaints. The mean differences between the groups, however, were extremely small (0.07 mm to 0.51 mm), and not all levels were statistically significant. The relative risk for an L5-S1 measurement 2 standard deviations below the mean was 1.4, yet the measurement explained less than 1% of the uncertainty in predicting complaints. No association was found between canal measurements and claims with extended work loss of greater than one month. The imprecision of the measurements and poor predictive ability indicate that B-scan ultrasonography, as used in this study, is of dubious screening value.