STUDY DESIGN.: Multicenter, prospective clinical series. OBJECTIVE.: To investigate the effect of preoperative bracing on postoperative outcome of posterior spine fusion with instrumentation for adolescent idiopathic scoliosis. SUMMARY OF BACKGROUND DATA.: Bracing is the standard of care for adolescent idiopathic scoliosis between 25° and 45°, yet the efficacy of bracing is questionable. It is important to evaluate the effect of bracing on outcomes in the adolescent idiopathic scoliosis population. METHODS.: We reviewed the outcomes of 281 before surgery braced and 328 before surgery nonbraced patients who underwent posterior spine fusion with instrumentation for adolescent idiopathic scoliosis before operation and at 2 years after operation using the Scoliosis Research Society instrument (SRS-30) and the Spinal Appearance Questionnaire. RESULTS.: At 2 years after operation, nonbraced patients demonstrated a greater improvement in the SRS-30 Pain domain score (0.23 vs. 0.08, P < 0.001), more improvement in back pain at rest (26.7% vs. 20.5%, P = 0.0009), and more improvement in back pain in the past 6 months (42.4% vs. 32.6%, P = 0.039) compared to braced patients. Also at 2 years after operation, nonbraced patients reported higher SRS-30 Activity domain scores (4.38 vs. 4.32, P = 0.031), Satisfaction domain scores (4.53 vs. 4.42, P = 0.007), and Total scores (4.27 vs. 4.35, P = 0.036) compared with braced patients. The 2-year Spinal Appearance Questionnaire scores showed that nonbraced patients reported a greater "decrease in importance" than braced patients in having "more even shoulders" (79.4% vs. 70.5%, P = 0.03), "more even hips" (74.6% vs. 71.6%, P = 0.042), and "more even ribs in back" (78.4% vs. 69.5%, P = 0.05). CONCLUSION.: Before surgery braced patients have more pain, lower activity levels, lower satisfaction, and lower total SRS-30 scores at 2 years after operation. Braced patients also have more "spine-specific" appearance concerns compared to nonbraced patients. These results suggest a negative impact of preoperative bracing on outcomes after posterior spinal fusion for adolescent idiopathic scoliosis. This "brace signature" should be taken into account when brace treatment is being considered.