This study examined the role of facet joint morphology in the etiology of both degenerative spondylolisthesis and isthmic spondylolysis. To this end, the axial facet joint morphology of the lower lumbar spine in a normal population and in populations of patients with spinal stenosis or degenerative spondylolisthesis at L4-5 and in patients with isthmic spondylolysis at the L5 level were characterized. Computed tomographic scans were digitized, defining the axial morphology of the normal facet joint at five stations from proximal to distal within the joint. Assessments were made of facet joint orientation, transverse articular dimension, depth of the articular surface, and shape of the articular surface at levels L3-4, L4-5, and L5-S1. There was a gradually more coronal orientation from proximal to distal among the stations at each level, and a maximal transverse articular dimension at the level of the superior endplate of the caudad vertebra. Minimal error in the recording process at this level. In addition to the maximal joint dimension, made this level the most representative of the overall morphology and most useful for further studies. At the L4-5 level, a significantly more sagittal facet orientation was found in the degenerative spondylolisthesis group when compared to both the normal population and spinal stenosis groups (P < 0.01). At L5-S1, the only significant morphologic difference between the normal population and the patients with isthmic spondylolysis was reduced transverse articular dimension. These results support the hypothesis that patients developing degenerative spondylolisthesis are predisposed to this by a developmental sagittal orientation of the L4-5 facet joints.