The discussion regarding factors that reliably predict the long-term surgical results in patients with lumbar spinal stenosis is still going on. This retrospective study analyses the relation between the dimensions of the dural sac and patients' clinical status before and after decompressive operations performed with or without additional discectomy. The type of surgery performed in 134 patients and the dural sac dimensions measured on postmyelograpic computed tomography in 100 of these patients were related to the Prolo scores before surgery and at follow-up (mean 46 months). The degree of dural sac compression correlated significantly with the patients' postoperative Prolo score and with the difference between the pre- and postoperative scores. The dural sac diameters predicted outcome after surgery more reliably than the preoperative Prolo scores. There was no statistically significant difference in the outcome when comparing patients with and without additional discectomy. The results presented suggest that the relief of symptoms after decompressive surgery for lumbar spinal stenosis correlates with the degree of the dural sac compression and that the simultaneous presence of disc herniation necessitating additional discectomy does not influence the postoperative outcome. However, these results have to be confirmed by prospective studies.