Purpose <p>This retrospective cohort study aimed to compare brace treatment outcomes in patients with adolescent idiopathic scoliosis (AIS) according to skeletal maturity using the distal radius and ulna (DRU) classification. The effectiveness of brace treatment in preventing curve progression across different stages of skeletal maturity, as defined by the DRU classification, remains unclear.</p> Methods <p>The study reviewed 69 patients with AIS treated with an underarm brace between 2014 and 2022 and followed them until skeletal maturity. Based on the DRU classification, the patients were stratified into three progression risk groups: high-risk (R6, R7), moderate-risk (R8), and low-risk (R9). Curve progression was defined as a ≥ 6° increase by skeletal maturity or surgery before maturity; progression requiring surgery was defined as a curve ≥ 50° at maturity or the need for spinal fusion. Prevention and surgery-requiring progression rates were compared among the three groups.</p> Results <p>Sixty-nine patients with AIS were included: 29, 28, and 12 in the high-, moderate-, and low-risk groups, respectively. Overall progression prevention rates in the high-, moderate-, and low-risk groups were 34.3%, 80%, and 100%, respectively (<i>p</i> &lt; 0.001). The high-risk group differed significantly from the other groups. Surgical-threshold prevention rates were 70.4%, 80.0%, and 100% in the high-, moderate-, and low-risk groups, respectively (<i>p</i> = 0.122). In the high-risk group, patients with Cobb angles ≥ 30° more frequently reached the surgical threshold.</p> Conclusion <p>These findings offer patients and families valuable guidance regarding curve control with brace treatments.</p>