OBJECTIVES: Flexible endoscopic septum division is an established treatment for Zenker's diverticulum (ZD); however, long-term outcome data are lacking. We aimed to evaluate the long-term efficacy of flexible endoscopic septal division (FESD) using the stag beetle knife for ZD and identify predictors of symptom recurrence. METHODS: Patients undergoing the procedure between 2013 and 2018 were prospectively enrolled. Procedures were performed by a single operator. Symptom severity pre- and postprocedure was recorded using the dysphagia, regurgitation, and complications scale. Symptom recurrence was defined as a total score > 1 after the index procedure. Time-to-event analyses were performed using Kaplan-Meier plots, with multivariable analyses performed using Cox regression models. RESULTS: Altogether, 65 patients (mean age 74.0 years, 60% male) were included. Previous stapling had been performed in 44.6% of patients. Over the mean posttreatment follow-up period of 19 months, 5.6% of the treatment naïve group and 34.5% of the recurrent group underwent repeated FESD (P = 0.003), with rates of symptom remission and improvement of 75.4% and 92.7%, respectively. Recurrence at 48 months was higher in patients with recurrent ZD (84.7%) than in treatment-naïve patients (10.7%). On multivariable analysis, recurrent disease (hazard ratio [HR] 20.8, P = 0.005) and younger age (HR 0.96/year, P = 0.047) were associated with symptom recurrence. CONCLUSIONS: In patients with treatment-naïve ZD, flexible endoscopic septal division is safe and provides durable symptom remission. However, in patients with poststapling recurrence, the risk of recurrence is high and time-dependent.