Up to now there have been two contradictory opinions in regard to contribution of lumbar facet in the development of disc degeneration and herniation. To reconfirm the clinical validity of facet arrangement in the development of early disc degeneration and herniation, this study was carried out, For this purpose, the relationship between lumbar unilateral disc protrusion and geometry of the facet joints in the transverse plane was investigated. In 35 cases of protrusion at the fourth disc and 19 cases of protrusion at the fifth disc, the geometry of the facet joints were measured on the computed tomographic sections taken 2mm intervals. The non-protruded discs and traumatized patient's disc of each level, were used as controls of the protruded discs of the same levels. The following results were obtained; 1. The facet angle showed much difference in the individual variance. Among the L4-5 disc group, the mean facet angle in the protruded side was 49±8.8°(29-70°), 48±8°(29-65°) in the opposite side, and 48+8°(30-65°) in the control group. In L5-Sl group each was 53±11°(35-79°), 51±7°(36-79°) and 52±10°(36-79°). 2. Transverse interfacet angle(TIFA) of the L4-5 disc group was 97-14°(59-125°) and of control group was 96±16°(66-127°) that. And those of L5-Sl group each were 104±19°(70-132°) and 104±19°(76-151°). 3. The angle difference between both sides in cases of facet asymmetry of the L4-5 disc group was 6.51±5.91°(0-30°), and of the control group was 4.95±3.88°(0-15°) that. And those of L5-Sl group was 8±6.41°(0-24°) and 6.18±6.64°(0-24°)and 6.18±6.64°(0-24°). When comparing the direction of disc herniation with the obliquity of the facet angle, herniation developed toward the more oblique side of the facet joint in 19 of 35 patients at the L4-5 disc while the L5-Sl disc herniation developed toward the more oblique side of the facet joint in 11 of 19 patients. 4. Among the L4-5 disc group, the mean moment arm angle in the protruded side was 22±4°(16-40°), 22±4°(15-35°), 22.4±4°(16-35°) in the opposite side and in control group. And those of L5-Sl group each was 26±4°(21-36°), 25±5°(17-35°) and 28±4°(16-35°). 5. Among the L4-5 disc group, the mean non-dimensional facet depth in the protruded side was 0.14±0.08°(0-0.36°), 0.15±0.08°(0-0.41°) in the opposite side and 0.18±0.06°(0.02-0.28°) in control group. And those of L5-Sl group were 0,12±0.07°(0.02-0.28°), 0.09±0.04°(0.02-0.2°) and 0.11±0.05°(0.02-0.28°). The facet angle, the transverse interfacet angle, the facet joint asymmetry, the moment arm angle and the non-dimensional facet depth showed no significant correlation, whether there was a disc protrusion or not.