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Background: Owing to an increased number of colonoscopy screenings, the incidence of diagnosed rectal neuroendocrine tumors (NETs) has also increased. Tumor size is one of the most frequently regarded factors when selecting treatment; however, it may not be the determinant prognostic variable. We aimed to evaluate oncological outcomes according to the treatment modality based on the size of rectal NETs. Methods: A retrospective analysis was performed on patients who were treated for rectal NETs between March 2000 and January 2016 at the Asan Medical Center, Seoul, Korea. Patients who underwent endoscopic removal, local surgical excision, and radical resection were included. The primary outcome was recurrence-free survival (RFS). Data were specified and analyzed following the 2019 World Health Organization classification (WHO). Results: A total of 644 patients were categorized under three groups according to the treatment modality used: endoscopic removal (n = 567), surgical local excision (n = 56), and radical resection (n = 21). Of a total of 35 recurrences, 27 were local, whereas eight were distant. The RFS rate did not differ significantly between the treatment groups in the same tumor-size group (≤ 1 cm group: P =.636, 1–2 cm group: P =.160). For T1 tumors, RFS rate was not different between local excision and radical resection (≤ 1 cm group: P =.452, 1–2 cm group: P =.700). Depth of invasion, a high Ki-67 index, and margin involvement were confirmed as independent risk factors for recurrence. Among patients treated with endoscopic removal, endoscopic biopsy was a significant factor for worse RFS (P <.001), while tumor size did not affect the RFS. Conclusion: The current guideline recommends treatment options according to tumor size. However, more oncologically important prognostic factors include muscularis propria invasion and a higher Ki-67 index. [ABSTRACT FROM AUTHOR]