Background and Objectives: In this retrospective cohort study, we investigate associations between the Hounsfield units (HU) value of upper instrumented vertebra (UIV) and proximal junctional kyphosis (PJK) after adult spinal deformity (ASD) surgery.Materials and Methods: The cohort consisted of 60 patients (mean age 71.7 years) who underwent long instrumented fusion surgery (≥6 vertebrae) for ASD with at least 1 year of follow-up. The preoperative bone mineral density (BMD) measured on DXA scans, the HU values at UIV and UIV+1, and the radiographic parameters were compared between the PJK and non-PJK groups. The severity of UIV fracture was assessed using a semiquantitative (SQ) grade.Results: PJK occurred in 43% of patients. No significant differences in patient age, sex, BMD, and preoperative radiographic parameters were observed between the PJK and non-PJK groups. The HU values of the UIV (103.4 vs. 149.0,p< 0.001) and UIV+1 (102.0 vs. 145.7,p< 0.001) were significantly lower in the PJK group. The cutoff values of HU at UIV and UIV+1 were 122.8 and 114.9, respectively. Lower HU values at UIV (Grade 1: 134.2, Grade 2: 109.6, Grade 3: 81.1,p< 0.001) and UIV+1 (Grade 1: 131.5, Grade 2: 107.1, Grade 3: 82.1,p< 0.001) were associated with severe SQ grade.Conclusions: Lower HU values at UIV and UIV+1 had a negative impact on signal incidence of PJK and were correlated with the severity of UIV fractures. Preoperative treatment of osteoporosis seems necessary for preoperative UIV HU values less than 120.