OBJECTIVE To define facet tropism (FT) for each motion segment of the lower lumbar spine for subjects without disc herniation, and to evaluate the significance of facet tropism in disc herniation. METHODS Sixty subjects were evaluated by measuring the facet angle (FA) of three lower lumbar motion segments with the use of computed tomography (CT) and a computer program. The subjects were divided into two groups: 33 without disc herniation and 27 with disc herniation at one or more levels of lower lumbar motion segments. The FA was defined as the angle of the facet and midsagittal lines. The difference between the right and left FAs at each motion segment was calculated. FT was defined as an angle difference larger than the mean and one standard deviation of the angle differences between the right and left FAs at each motion segment in the group without disc herniation. METHODS Statistical significance of the bilateral angle difference of each lower lumbar motion segment between the two groups was analyzed by the two-sample t test. Using the defined angle difference for FT, the incidence of FT in the two groups at each motion segment was analyzed by the Fisher's exact test. RESULTS FT was defined for the 33 subjects without disc herniation on CT scans as an angle difference larger than 12 degrees at L3-L4, 15 degrees at L4-L5, and 12 degrees at L5-S1, approximately. There was no statistical significance in the bilateral FA difference (p > .05) and incidence of FT (p > .05) at each segment between the two groups. CONCLUSIONS This study did not show that facet joint tropism plays a significant role in disc herniation in the lower lumbar spine.