Interpedicular distance, interfacet distance, midsagittal diameter, and cross-sectional area at the upper aspect of the fifth lumbar spinal canal were measured from the computed tomographic (CT) scans of the spine performed in a period of 1 year. The patients were divided into four groups. Group I (25 patients) was the normal control group. Group II comprised 29 symptomatic patients who were thought to have an L4–L5 herniated nucleus pulposus (HNP) by CT and did not undergo surgery. Group IIIA was made up of 24 patients who underwent an L4–L5 discectomy and had favorable results, and Group IIIB (3 patients) included those who failed to improve following surgery. The symptomatic patient with an L4– L5 HNP by CT who did not undergo operative treatment had a mean canal size as measured by midsagittal diameter and cross-sectional area that was smaller (P < 0.05) than in a normal control group. In the patients who required an L4–L5 discectomy, these same measurements were smaller (P<0.001) when compared with the nonoperative group. In the three failure patients, all four measurements were significantly smaller than in patients in Group IIIA with the interfacet distance and the cross-sectional area differences being the greatest (P < 0.001). The patients who are likely to undergo operative treatment have a midsagittal diameter that is < 1.6 cm and a cross-sectional area that is < 2.5 cm.2 In the congenitally small canal the cross-sectional area more accurately reflects the degree of stenosis and a cross-sectional area that is < 2 standard deviations from the normal or < 2 cm2 is considered to represent congenital spinal canal stenosis.