The cross-sectional area of the lumbar dural tube was calculated from transverse-slice lumbosacral magnetic resonance images (MRI) using the simple geometric formulas and computerized digitizer in 51 patients with low-back pain only, 79 with mainly radicular symptoms, and 53 with intermittent claudication, and in 39 controls with confirmed symptomatic lesions at a nonlumbar level or of a nonspinal organ. Findings suggesting developmental narrowing of the spinal canal were found not only in the intermittent claudication group, but also in the radicular pain group. Cross-sectional area of < 100 mm2 at more than two of three (L2/3, L3/4, and L4/5) intervertebral levels was highly associated with the presence of intermittent claudication. The cross-sectional area value obtained with the simplified geometric formulas was highly correlated with that calculated with the digitizer, indicating that this simple method can be used with MRI in outpatient clinics for the rapid determination of the most stenotic portion of the dural tube.