We report a prospective study analysing whether possible factors predisposing to degenerative spondylolisthesis (DS) must be present concomitantly in order to cause vertebral slipping. Standard and flexion-extension radiographs were obtained from 27 patients with DS and 27 without spondylolisthesis. The level of the intercrestal line, the lumbosacral angle, the presence of sacralization of L5 and vertebral motion at the L4-L5 level were assessed. Facet joint orientations were measured on CT scans. Only facet joint orientation and vertebral motion at the spondylolisthetic level were significantly different in patients with DS compared with controls. Facet joints were oriented more sagittally both at the spondylolisthetic level and at the levels above and below. An inverse linear correlation was found between the sagittal orientation of facet joints and the mobility of the slipped vertebra. Abnormal sagittal orientation of facet joints and hypermobility of the spondylolisthetic vertebra appear to play major roles among possible factors predisposing to DS. Both factors should be considered in the planning of surgical treatment.