Background: Proximal junctional kyphosis (PJK) is a common complication of long spinal fusion. The relationship between body mass index (BMI) and PJK is controversial. 
Objectives: This study aimed to evaluate the relationship between patient BMI and PJK prevalence in patients undergoing long spinal fusion. 
Methods: This retrospective study included 76 patients who underwent long spinal fusion (≥5 levels or pelvic fixation) for degenerative canal stenosis with a minimum 24-month follow-up. PJK was defined as ≥10° increase in Cobb angle between the UIV inferior endplate and the UIV+2 superior endplate. 
Results: PJK occurred in 26 patients (34.2%) at early follow-up (≤6 months) and in 18 patients (23.7%) at late follow-up (>6 months). Mean BMI was significantly higher in the early PJK group (29.6 vs 23.4 kg/m², P<0.001). BMI was an independent predictor of early PJK (OR=1.32, 95% CI, 1.18%, 1.48%, P<0.001). Overweight patients (BMI>25) had a 69.6% early PJK rate vs 20.5% in normal-weight patients. Surgically treated early PJK patients had higher mean BMI (30.8 vs 28.4 kg/m², P=0.023).
Conclusion: Higher BMI is significantly associated with early PJK following long spinal fusion, particularly in the first 6 months postoperatively.