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Purpose: To determine the incidence rate and identify predictive factors for interval appendectomy after non-operatively treated complicated appendicitis., Methods: Single-center retrospective cohort study conducted between January 2008 and June 2017. Adult patients with acute appendicitis were identified. Patients with complicated appendicitis initially treated non-operatively were included. Outcomes included abscess rate on imaging, results of additional imaging during follow-up, incidence rate of and surgical indications for interval appendectomy, and outcomes of histological reports., Results: Of all adult patients with acute appendicitis (n = 1839), 9% (170/1839) was initially treated non-operatively. Median age of these patients was 55 years (IQR 42-65) and 48.8% (83/170) were men. In 36.4% (62/170) of the patients, an appendicular abscess was diagnosed. 62.4% (106/170) did not require subsequent surgery (no interval appendectomy group) and in 37.6% (64/170), an interval appendectomy was performed (interval appendectomy group). Median follow-up was 80 weeks (17-192) and 113 weeks (34-246), respectively. Most frequent reason to perform subsequent surgery was recurrent appendicitis (45% (29/64)). Increasing age was significantly associated with a lower risk of undergoing interval appendectomy (OR 0.7; CI 0.6-0.89); p = 0.002). In the interval appendectomy group, appendicular neoplasm was found in 11% (7/64) of the patients, in contrast to 1.5% (25/1669) of the patients that had acute surgery (p < 0.001)., Conclusions: One out of three patients non-operatively treated for complicated appendicitis required an interval appendectomy. The incidence of appendicular neoplasms was high in these patients compared with those that had acute surgery. Therefore, additional radiological imaging following non-operatively treated complicated appendicitis is recommended.