Study DesignMulticenter, prospective study of consecutive adult spinal deformity (ASD) patients.ObjectiveTo Validate Schwab's classification accuracy for surgical indication, and to evaluate a simplified sagittal modifier.Summary of Background DataThe SRS-Schwab Radiologic Classification based on clinical impact parameters, offers 27 different sagittal classification possibilities regarding sagittal vertical alignment (SVA), pelvic tilt (PT), and pelvic incidence-lumbar lordosis (PILL). The high number of classification possibilities makes it complex to use.MethodsInclusion criteria were ASD patients, presenting at least 1 criteria: Cobb >= 20 degrees, SVA >= 5 cm, thoracic kyphosis >= 60 degrees, or PT >= 25 degrees. A total of 1,004 patients (410 nonoperative and 594 operative) were classified regarding SVA, PT, and PI-LL (0, +, ++), and 27 possibilities were identified. Categories were formed by adding the number of + signs, considering PT, SVA, and PI-LL. Three specific categories were identified: Aligned: 0 +; Moderate deformity: 1 to 3+; and Severe deformity: 4 to 6+. A chi -square test was performed for surgical indication (operated or not) and an analysis of variance was performed to evaluate the relationship between categories and Oswestry Disability Index (ODI). Probability <.05 was considered significant.ResultsSignificant differences for HRQoL scores and surgical indication were found in the 27 sagittal parameter possibilities. For nonoperative patients, 230 (56.1%) were classified as aligned, 145 (35.4%) as moderate, and 35 (8.5%) as severe. For operative patients, there were 200 (33.7%), 215 (36.2%), and 179 (30.1%) in each respective subgroup. For HRQoL scores and surgical indication, no significant differences were found within each category, but significant differences were found when comparing the subgroups.ConclusionsDespite the correlation between SRS-Schwab classification and surgical indication, it is complex to use, with a total of 27 possibilities regarding sagittal modifiers. This simplification into three categories offers more readability, without losing any significant information, and could replace Schwab sagittal modifiers. In association with other parameters, they could be used for decision-making.Level of EvidenceLevel II.