Endoscopic mucosal resection is effective in the treatment of flat or laterally developed colorectal tumors larger than 20 mm in diameter, but for complex lesions, such as laterally developed tumors of the large intestine including the ileocecal valve, the safety, efficacy, and clinical prognosis of endoscopic mucosal resection treatment are limited because these patients are more likely to choose surgery. This is a single-center, prospective, observational cohort study of patients who presented for endoscopic mucosal resection for lateralized developmental tumors of the large intestine, including the ileocecal valve. Information collected included patient age, anesthetic classification, previous endoscopic mucosal resection, lesion location, size, morphology, Paris classification, success of endoscopic mucosal resection, degree of submucosal fibrosis, histological staging, and whether the tumor was progressive. The first follow-up colonoscopy was performed 4 to 5 months after endoscopic mucosal resection, and the second one was performed 16 months after surgery. High-resolution white light endoscopy and narrow-band imaging were chosen for the review, and biopsies were taken at the surgical scar to clarify whether there was recurrence. The results of the study found that resection of lateralized developmental tumors of the large intestine including the ileocecal flap was more difficult, with lower rates of whole-block resection and successful surgical resection compared with lesions at other sites, but the complication rates were similar in both groups. These studies suggest that lesion involvement of the ileum or both the upper and lower lip is an independent risk factor for failure of endoscopic mucosal resection and the need for surgical treatment, whereas lesion size and involvement of the entire circumference of the ileocecal valve are not relevant. Therefore, the authors concluded that endoscopic mucosal resection is a safe and effective treatment for lateralized developmental tumors of the large intestine, including the ileocecal valve, with few complications, allowing patients to avoid invasive surgical procedures.