Ninety-three patients with lumbar central canal stenosis were prospectively evaluated to determine whether the posterior epidural fat was an important component in the pathogenesis of thecal sac compression. Results of this study indicated that, in patients with acquired stenosis secondary to facet arthrosis, posterior compression of the thecal sac by epidural fat was identified. This finding was not observed in patients with stenosis secondary to anterior column pathology or in patients with developmentally large facets.