To conduct a comprehensive study of postendoscopic adverse events (AEs) in children, categorizing them by the level of intervention required. Previous studies of endoscopic AE in children have focused on intraprocedural and short-term outcomes, such as bleeding, perforation, and infection, and may underestimate the incidence of AEs. Prospective observational study tracking AEs in pediatric patients within 72 hours of an endoscopic procedure. Single-center study performed at an academic, tertiary care, free-standing children's hospital over a 48-month period, from July 2010 through June 2014. Of 9577 pediatric endoscopic procedures, cases identified as having an AE during or following endoscopy were subject to additional chart review for abstraction of relevant data. Type, severity, and subsequent interventions of AEs were the primary outcome measures. Events were categorized by severity grade, with grade 1, no intervention; grade 2, outpatient evaluation; grade 3, hospitalization or repeat endoscopy; grade 4, surgery or intensive care unit admission; and grade 5, death. In total, 249 endoscopic AEs were recognized of 9577 procedures performed, yielding a rate of 2.6%. Of these, 160 were identified to result in medical evaluation and costs (≥grade 2) for a rate of 1.7%. Rates of endoscopic AE after advanced or interventional endoscopic procedures were higher, with 65 of 1167 events, resulting in a total AE rate of 5.6% and a rate of 4.4% for AE ≥ grade 2. There were 10 cases of significant bleeding (0.1%), 9 cases of infection (0.09%), and 12 cases of perforation (0.13%), primarily occurring with advanced/interventional procedures. Single-center study, lack of standardized criteria for ED referral. AEs presenting within 72 hours of endoscopy and resulting in medical intervention, occur more commonly than previously recognized in children. Standardized postendoscopy surveillance systems and definitions of AEs are needed.