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STUDY DESIGN A retrospective study of surgical outcomes. OBJECTIVE This study aimed to investigate the preoperative risk factors for proximal junctional kyphosis (PJK) in adult spinal deformity (ASD) surgery. SUMMARY OF BACKGROUND DATA The cause of PJK is still unclear, although some risk factors have been reported in ASD surgery. METHODS A total of 185 patients who were followed up for more than two years and underwent ASD surgery were recruited. PJK was defined as a proximal junctional angle ≥20° or reoperation due to PJK within two years after surgery. These patients were divided into PJK and non-PJK groups. Whole-spine standing radiography was performed before and immediately, one year, and two years after the surgery. RESULTS The PJK and non-PJK groups comprised 58 and 127 cases, respectively. The incidence of PJK demonstrated significant differences according to preoperative thoracic kyphosis (TK): 37% (TK ≤19°), 33% (TK 20°-29°), 9% (TK 30°-39°), 32% (TK 40°-49°), and 41% (TK ≥50°) (p < .05). Logistic regression analysis suggested that the amount of change in TK before and just after the surgery (ΔTK) was a significant risk factor for PJK (p < .001; odds ratio 1.062, 95% confidence interval 1.029-1.097). CONCLUSION ΔTK was less in the TK group of 30°-39° because the TK of patients who underwent ASD surgery converged to 34.5° just after surgery. Consequently, a lower or higher TK was likely to result in a large ΔTK just after surgery. Therefore, patients who had an optimal TK (30°-39°) had a lower risk of PJK. LEVEL OF EVIDENCE Level IV.