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Study Design: Retrospective cohort study. Objective: Analysis of postoperative sagittal alignment of the unfused spine is lacking in patients with adult spinal deformity (ASD). The present study aims to evaluate the efficacy of the whole spine full-flexion lateral radiograph to predict the reciprocal change of the unfused spine after correction surgery. We hypothesized that the novel parameter (T1-UIV angle: angle between the upper vertebral endplate of the T1 and the upper vertebral endplate of the upper instrumented vertebra) of the preoperative whole spine full-flexion lateral radiograph is similar to that of the postoperative lateral radiograph if the patient has the ideal sagittal alignment. Methods: Twenty-six ASD patients who underwent correction surgery with a minimum 2-year follow-up were enrolled and separated into the Ideal and Non-Ideal groups according to the Scoliosis Research Society (SRS)-Schwab classification of the final follow-up radiograph. Radiographic parameters, including T1-UIV of the preoperative whole spine full-flexion lateral radiograph, were obtained. Results: Thirteen patients were included in the Ideal group and 13 were in Non-Ideal group. Preoperative T1-UIV of the whole spine full-flexion lateral radiograph exhibited significant correlations with the T1-UIV angles of the postoperative and final follow-up radiographs ( r = 0.64, P < .01, y = 0.800 x + 8.012, and r = 0.69, P < .01, y = 0.857 x + 2.960, respectively). Interestingly, this correlation was stronger for the Ideal group ( r = 0.77, P < .01, y = 1.207 x − 1.517, and r = 0.89, P < .01, y = 0.986 x + 0.694, respectively). Conclusion: A novel radiographic strategy (T1-UIV of preoperative the whole spine full-flexion lateral radiograph) could estimate the postoperative alignment of the unfused spine correctly.