Background: Cast treatment for infantile idiopathic scoliosis patients ultimately corrects deformity in varying amounts. As the reasons for these differential outcomes are not fully elucidated, the aim of this study was to identify clinical and radiographic variables correlated with better cast correction. Methods: Patients in the Children's Spine Study Group and Growing Spine Study Group registries who underwent cast treatment for idiopathic scoliosis between 2005 and 2013 with 1-year minimum follow-up were included. Data including major curve and rib-vertebra angle difference before cast, initial in-cast application, after cast treatment, and at most recent follow-up were collected. Univariable and multivariable regression analyses were used to identify factors associated with lower major curves at most recent follow-up. Results: A total of 68 patients were identified and followed for a mean of 2.5 (range, 1.1 to 5.4) years after cast treatment. Cast treatment lasted an average of 16.7 months, with a median of 6 cast applications (range, 2 to 19). Twenty-five subjects (37%) had a most recent major curve <15 degrees (success), whereas 43 had a curve that was >15 degrees (unresolved). Multivariable linear regression determined that younger age (P=0.02), smaller precast major curve (P<0.001), and greater percent major curve correction in initial cast (P=0.006) were associated with smaller major curve at most recent follow-up. Multivariable logistic regression determined that success patients were younger than unresolved patients (average age, 1.4 vs. 2.1 y; P=0.003), and had smaller in-cast major curves after initial cast application (average, 18 vs. 27 degrees; P=0.002). Conclusions: Infantile idiopathic scoliosis patients casted at an earlier age, with smaller major curves, and greater percent major curve correction in initial cast have the best prognosis. Patients' percent major curve correction, which may represent curve flexibility and/or cast quality, is a predictor of treatment success when age and precast major curve are also taken into account. Level of Evidence: Level III - retrospective study.