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Background The nerve root sedimentation sign is a magnetic resonance (MR) sign, shown to be present in central lumbar spinal stenosis. The lack of sedimentation of the nerve roots to the dorsal part of the dural sac is consistent with the positive nerve root sedimentation sign. Purpose To validate the reliability of the nerve root sedimentation sign in diagnosis of different grades of lumbar spinal canal stenosis. Material and Methods This study was a retrospective review of 101 consecutive MR imaging (MRI) studies obtained on patients with clinically suspected lumbar canal stenosis. Based on the minimum anteroposterior (AP) diameter of the dural sac the study sample was classified into two groups: a group with morphological lumbar spinal stenosis; and the group of patients free from stenosis (AP > 12 mm). Patients with stenosis were further subclassified based on its severity: severe stenosis (AP ≤ 10 mm); and moderate stenosis (AP > 10 mm to ≤ 12 mm). Results Positive sedimentation sign was identified in 81% of patients with severe lumbar spinal stenosis and 14% of patients with moderate stenosis. No patients without lumbar spinal stenosis had a positive nerve root sedimentation sign. Of patients with a positive nerve root sedimentation sign, 89% presented with neurological claudication. Conclusion The nerve root sedimentation is a useful tool for identification of patients with both severe clinical and morphological lumbar spinal stenosis; however, its performance in the diagnosis of patients with moderate morphological spinal stenosis is poor.