BACKGROUND CONTEXT Violation of the posterior soft tissues is believed to contribute to the development of proximal junctional kyphosis (PJK). Clinical and biomechanical studies have suggested that augmentation of the posterior ligamentous structures (PLS) may help prevent PJK. PURPOSE To determine whether augmentation of the PLS from the UIV-1 to UIV+1 prevents PJK. STUDY DESIGN/SETTING Single-surgeon retrospective cohort study. PATIENT SAMPLE One hundred fifty adult spinal deformity patients. OUTCOME MEASURES PJK (defined as a proximal junctional angle ≥20°) at final follow-up. METHODS We conducted a retrospective cohort study of 150 adult spinal deformity patients who underwent ≥5 level fusions to the pelvis by a single surgeon between 2014 and 2017. Exclusion criteria included neuromuscular or congenital scoliosis (18 patients), or follow-up less than 1 year (24 patients), leaving 108 patients available for final analysis. Patients were divided into two groups: PLS+ patients had reconstruction of the PLS between UIV+1 and UIV-1 with a surgical nylon tape while PLS- patients did not. Demographics, surgical data, and sagittal alignment parameters were compared between the cohorts. The primary outcome of interest was the development of PJK at final follow-up (1 or 2 years). A subgroup propensity match and backward stepwise logistic regression model were utilized to control for surgical and radiographic differences in the cohorts. RESULTS Thirty-one patients (28.7%) were PLS+. There were no differences with regard to preoperative or final sagittal alignment parameters, number of levels fused (10.7±3.8 vs 9.7±3.8, p=0.331), and BMI (24.9±5.2 vs 27.0±6.9 kg/m2, p=0.143), though the PLS+ cohort was older (64.1±10.4 vs 51.3±21.4 years, p=0.002) and had greater 6-week corrections in sacral slope (SS), pelvic tilt (PT), pelvic incidence-lumbar lordosis (PI-LL), C7-S1 sagittal vertical axis (C7 SVA), and T1 pelvic angle (T1PA), p 0.05 for 6-week changes in SS, PT, LL, PI-LL, C7 SVA, T1PA, thoracic kyphosis, thoracolumbar kyphosis, and cervical lordosis), PLS augmentation was still not associated with a lower rate of PJK (29% PLS+ vs 38.7% PLS-, p=0.367). In our multivariate analysis, only increased sagittal malalignment and failure to restore sagittal balance were retained as having independent associations with PJK, while PLS+ was not retained in the model. CONCLUSIONS Even after controlling for relevant variables such as age, preoperative alignment, and degree of correction, PLS reinforcement at the UIV+1 did not reduce the incidence of PJK at 1 year. Our findings emphasize the multifactorial nature of PJK and the difficulty of preventing PJK with a single surgical technique FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.