Objective: The reported incidence of complications and/or adverse events (AEs) following spine surgery varies greatly. A validated, systematic, reproducible reporting system to quantify AEs was used in 2 prospective cohorts, from 2 spine surgery centers, conducting either complex or purely degenerative spine surgery; in a comparative fashion. The aim was to highlight the differences between 2 distinctly different prospective cohorts with patients from the same background population. Methods: AEs were registered according to the predefined AE variables in the SAVES (Spine AdVerse Events Severity) system which was used to record all intra-and perioperative AEs. Additional outcomes, including mortality, length of stay, wound infection requiring revi-sion, readmission, and unplanned revision surgery during the index admission, were also registered. Results: A total of 593 complex and 1,687 degenerative procedures were consecutively included with 100% data completion. There was a significant difference in morbidity when comparing the total number of AEs between the 2 groups (p < 0.001): with a mean number of 1.42 AEs per patient (n = 845) in the complex cohort, and 0.97 AEs per patient (n = 1,630) in the degenerative cohort. Conclusion: In this prospective study comparing 2 cohorts, we report the rates of AEs related to spine surgery using a validated reproducible grading system for registration. The rates of morbidity and mortality were significantly higher following complex spine surgery compared to surgery for degenerative spine disease.