The objective of this study was to evaluate the influence of decreased dural sac cross-sectional area and baseline clinical parameters on the outcome of patients treated surgically or conservatively for lumbar spinal stenosis. Computed tomography or magnetic resonance imaging scans of 37 patients were digitized and the dural sac cross-sectional area was calculated. This parameter and baseline clinical, socioeconomic, and anthropometric data of the patients were correlated with 1-year and 2-year follow-up data. The decrease in dural sac cross-sectional area negatively affected walking capacity on follow-up controls in patients treated conservatively, whereas such a relation was not observed among surgically treated patients. Female sex was the main parameter that worsened the global outcome of degenerative lumbar spinal stenosis, particularly after surgical treatment.