Background <p>The extent to which anal and anorectal tumors are missed during endoscopic examinations has not been thoroughly investigated. This study investigated the clinical features of tumors that were missed during endoscopic examinations.</p> Aim <p>To investigate and analyze the endoscopic and clinical features of missed anal and anorectal tumors.</p> Methods <p>All patients diagnosed with anal and anorectal tumors between 2014 and 2024 at West China Hospital were included in this retrospective cohort analysis. Demographic data, tumor characteristics, endoscopic findings, and missed diagnosis records were extracted from electronic medical records and telephone follow-up. A descriptive and error-classification analysis was performed to identify the reasons for missed diagnoses during endoscopy.</p> Results <p>Descriptive and error-classification analysis among the 83 enrolled patients, 10 (7 with adenocarcinoma and 3 with squamous cell carcinoma) had undergone previous endoscopic examinations with negative diagnoses, resulting in a missed detection proportion of 12.0%. Six missed cases with 15 prior colonoscopies in which the lesions were overlooked had complete medical records. The mean number of endoscopic procedures performed prior to the definitive diagnosis was 2.5 (range: 1–5), with an average diagnostic delay of 43.4 months (range: 1–82). The causes of missed diagnoses included exposure errors (<i>n</i> = 8), judgment errors (<i>n</i> = 6), and biopsy errors (<i>n</i> = 1). The proportion of endoscopic rectal retroflexion to observe anus was 20% .</p> Conclusion <p>The missed diagnosis proportion of anal and anorectal tumors was high during endoscopic examination. Comprehensive endoscopic observation and enhanced recognition of the endoscopic features of anal tumors are essential.</p>