© 2020 Elsevier Inc.Background: The minimum clinically important difference (MCID), an important concept to evaluate the effectiveness of treatments, might not be a single “magical” constant for any given health-related quality of life (HRQoL) scale. Thus, we analyzed the effects of various factors on MCIDs for several HRQoL measures in an adult spinal deformity population. Methods: Surgical and nonsurgical patients from a multicenter adult spinal deformity database who had completed pretreatment and 1-year follow-up questionnaires (Core Outcome Measures Index [COMI], Oswestry Disability Index [ODI], Medical Outcomes Study 36-item short-form questionnaire, 22-item Scoliosis Research Society Outcomes questionnaire, and an anchor question of “back health”–related change during the previous year) were evaluated. The MCIDs for each HRQoL measure were calculated using an anchor-based method and latent class analysis for the overall population and subpopulations stratified by age, gender, and baseline scores (ODI and COMI) separately for patients with positive versus negative perceptions of change. Results: Patients with a baseline ODI score of <20, 20–40, and >40 had an MCID of 2.24, 11.35, and 26.57, respectively. Similarly, patients with a baseline COMI score of <2.75, 2.8–5.4, and >5.4 had an MCID of 0.59, 1.38, and 3.67 respectively. The overall MCID thresholds for deterioration and improvement were 0.27 and 2.62 for COMI, 2.23 and 14.31 for ODI, and 0.01 and 0.71 for 22-item Scoliosis Research Society Outcomes questionnaire, respectively. Conclusions: The results from the present study have demonstrated that MCIDs change in accordance with the baseline scores and direction of change but not by age or gender. The MCID, in its current state, should be considered a concept rather than a constant.