STUDY DESIGN: Retrospective review of 363 patients who underwent posterior lumbar fusion (PLF). OBJECTIVE: Adjacent segment pathology (ASP) is a major and common event in patients who have undergone PLF. The objective of this study is to determine if ASP is due to accelerated processes following fusion or to pre-existing degeneration. SUMMARY OF BACKGROUND DATA: ASP is defined as degenerative changes that occur 1-2 levels above or below the site of fusion in patients. The etiology of ASP is a topic of debate. METHODS: Preoperative MRIs of 363 individuals who underwent PLF within L2-L3, L3-L4, and L4-L5 at the University of Kentucky between 2010 and 2020 were assessed for evidence of pre-existing degeneration. Measures of degeneration included Pfirrmann grade, modified Pfirrmann grade, disc height, and facet hyperintensity width. Demographic measures, including age, sex, smoking status, and BMI were also assessed. RESULTS: Throughout the follow-up period, 30.0% of patients evaluated were found to have ASP. 83.7% of these happened at the level below and 16.3% happened at the level above the fusion. Paired sample t testing indicated that only disc height was significantly different in the adjacent levels in those who developed ASP. There was no significant difference between the 2 levels for Pfirrmann grade, modified Pfirrmann grade, and facet T2 hyperintensity. Among degenerative measures, only disc height was different (lower) in the level below PLF compared with above, before fusion. Age, sex, and smoking status were not significantly different between those who developed ASP and those who did not (P=0.68, 0.81, 0.23, respectively). CONCLUSIONS: Analysis suggests that in patients undergoing PLF, pre-existing degeneration plays an insignificant role in the development of ASP, and that postoperative acceleration of degenerative changes still represents the primary etiology of ASP. LEVEL OF EVIDENCE: Level III.