The aim of our study is to evaluate the results and effectiveness of bilateral decompression via a unilateral approach in the treatment of degenerative lumbar spinal stenosis. We have conducted a prospective study to compare the midterm outcome of unilateral laminotomy with unilateral laminectomy. One hundred patients with 269 levels of lumbar stenosis without instability were randomized to two treatment groups: unilateral laminectomy (Group 1), and laminotomy (Group 2). Clinical outcomes were assessed with the Oswestry Disability Index (ODI) and Short Form-36 Health Survey (SF-36). Spinal canal size was measured pre- and postoperatively. The spinal canal was increased to 4-6.1-fold (mean 5.1 +/- SD 0.8-fold) the preoperative size in Group 1, and 3.3-5.9-fold (mean 4.7 +/- SD 1.1-fold) the preoperative size in Group 2. The mean follow-up time was 5.4 years (range 4-7 years). The ODI scores decreased significantly in both early and late follow-up evaluations and the SF-36 scores demonstrated significant improvement in late follow-up results in our series. Analysis of clinical outcome showed no statistical differences between two groups. For degenerative lumbar spinal stenosis unilateral approaches allowed sufficient and safe decompression of the neural structures and adequate preservation of vertebral stability, resulted in a highly significant reduction of symptoms and disability, and improved health-related quality of life.