<jats:sec> <jats:title>Study Design.</jats:title> <jats:p>Retrospective analysis of prospective observational cohort</jats:p> </jats:sec> <jats:sec> <jats:title>Objective.</jats:title> <jats:p>This study assessed the difference in health-related quality of life (HRQOL) between participants with a mild to moderate adult sagittal deformity (ASD) (sagittal vertical axis [SVA] ≤9.5 cm) and those with a marked deformity (SVA >9.5 cm). We also evaluated predisposing factors for a marked deformity.</jats:p> </jats:sec> <jats:sec> <jats:title>Summary of Background Data.</jats:title> <jats:p>Sagittal imbalance is closely associated with HRQOL for the patient. However, how the effect changes depending on the degree of imbalance has not been fully evaluated. The understanding of the predisposing factor associated with marked deformity also lacks.</jats:p> </jats:sec> <jats:sec> <jats:title>Methods.</jats:title> <jats:p>A total of 124 elderly persons with a stooping posture were enrolled. Questionnaires related to HRQOL were administered. Sagittal alignment parameters and pelvic parameters were measured with a whole spine x-ray. Lumbar spine magnetic resonance imaging was used to assess the presence of pathologic conditions, muscle quality and quantity. Multivariate logistic regression analysis was conducted to analyze potential risk factors.</jats:p> </jats:sec> <jats:sec> <jats:title>Results.</jats:title> <jats:p>Marked ASD was associated with female sex, lower height and weight, and osteoporosis (<jats:italic toggle="yes">P</jats:italic>  <jats:italic toggle="yes"><</jats:italic> 0.05). Back pain (assessed by a visual analogue scale) and the Oswestry Disability Index were significantly higher in the marked deformity group (<jats:italic toggle="yes">P</jats:italic> = 0.012, 0.002, respectively). Multivariate logistic regression analysis showed significant relationships between the following parameters and marked deformity: preexisting compression fracture (odds ratio [OR] = 7.793; 95% confidence interval [CI], 1.527–39.768), severe L5/S1 Pfirrmann disc degeneration grade (OR = 1.916; 95% CI, 1.086–3.382), and lower quantities of multifidus and psoas muscles (OR = 0.994, 0.997; 95% CI, 0.991–0.998, 0.994–0.999, respectively).</jats:p> </jats:sec> <jats:sec> <jats:title>Conclusion.</jats:title> <jats:p>Participants with a marked ASD showed different features from those with a mild to moderate ASD. This study also implies that anatomical factors, including the vertebrae, intervertebral discs, and paraspinal muscles, synergistically contribute to progression into marked deformity.</jats:p> <jats:p> <jats:bold>Level of Evidence:</jats:bold> 3</jats:p> </jats:sec>