To assess the value of computed tomography in degenerative lumbar spinal stenosis, postoperative computed tomography was compared to preoperative studies and other radiographic modalities in 20 patients, ten with an excellent surgical result and ten with a poor result. A close correlation was found between moderate to severe residual stenosis on postoperative CT and surgical failure. The most common cause of failure was inadequate unroofing of the lateral gutters. Computed tomography with multiplanar reconstructions detected stenosis not only in the central canal and lateral recesses, but in the neuroforaminal as well. Central canal stenosis corresponded to a marked deformity of complete block on myelography. Lateral recess stenosis was shown more often by computed tomography than by blunting of the nerve root sheaths on the myelograms. Neuroforaminal encroachment seen on computed tomography sagittal reconstructions was not demonstrable by myelography. Degenerative spondylolisthesis, a special stenotic condition, produced a characteristic "double margin" sign on computed tomography and the critical stenosis of the lateral recesses was demonstrated. This study suggests that the superior definition of the sites of degenerative spinal stenosis provided by computed tomography should lead to higher surgical success rates.