A prospective, comparative study on clinical, radiographic, and health-related quality of life (HRQoL) outcomes in adolescents with acute spondylolysis treated with a rigid thoracolumbar orthosis (Boston brace) or with a placebo (elastic lumbar support) with a 2-year follow-up time. To compare outcomes of acute adolescent spondylolysis treated with a rigid thoracolumbar orthosis or a placebo with a 2-year follow-up time. The benefits of the use of rigid orthosis for treatment of spondylolysis and achieving bony union of spondylolysis remains unclear. Sixty consecutive patients with acute spondylolysis were prospectively enrolled. Three patients were excluded from analysis as they did not fulfill inclusion criteria. First 14 patients were randomized and the remaining 46 chose treatment method themselves. Treatment time was four months and follow-up time was two years. HRQoL was measured using Scoliosis Research Society-24 (SRS-24) outcome questionnaire. The primary outcome was the HRQoL at 24 months and whether treatment type, bony union of the spondylolysis or development of spondylolisthesis affected it. Thirty (30/57) patients were treated with a Boston brace and twenty-seven (27/57) patients with a placebo. The bony union rate of spondylolysis did not differ between study groups (20/30 vs 17/27, respectively, P=0.789). The HRQoL did not differ between treatment groups in the SRS-24 domains through follow-up (P>0.05 for all). Five patients (5/57) developed spondylolisthesis (mean slip 4.2mm) during two-year follow-up time. Non-union of the spondylolysis predicted development of spondylolisthesis (P=0.005), but treatment type did not affect it (P>0.05). Two years after treatment patients who had bony union had higher SRS-24 total (P=0.005) and satisfaction domain (P<0.001) compared to patients with non-union. A rigid brace is not necessary for treatment of acute spondylolysis. Achieving bony union of adolescent spondylolysis is desirable as their HRQoL is higher at two years. II.