In the last two decades, the concept of spinal stenosis and its treatment by surgical decompression has been widely accepted. Complications such as olisthy, disc rupture, facet fracture, and intractable back pain began to appear postoperatively, suggesting instability as their cause. A retrospective study of 344 patients treated surgically for lumbar stenosis revealed a 17% reoperation rate for complications resulting from obvious or suspected instability. Sixteen cases of postdecompression olisthy, 14 cases of fresh disc herniation, and 27 cases of intractable back pain required further surgery. Preoperative indicators of potential instability are degenerated discs as evidenced by traction spurs or diminished disc height, olisthy, and scoliosis or asymmetrically narrowed discs. Total facetectomy and pars excision at surgery destabilize the spine and must be added to the preoperative risk factors for instability. Calcified annulus, capsule and ligamentum flavum, or complete disc resorption may offer some protection from postoperative instability. The level of instability may be preselected by the proximity to the intercrestal line. It is recommended that during surgical decompression for spinal stenosis, the posterior elements be spared as much as possible to avoid instability after surgery. Factors suggesting instability noted preoperatively or decompression which produces instability suggest that fusion should be combined with decompression. Spinal fusion is the treatment for postoperative instability.