BACKGROUND: Endoscopic submucosal dissection (ESD) enables direct submucosal dissection so even large early stage GI tumors can be resected en bloc. Colorectal ESD is technically more difficult, however, and there is an increased risk of complications such as perforation and bleeding compared with gastric ESD. As a result, further refinements are required in this procedure. OBJECTIVE: Our purpose was to evaluate thin endoscope-assisted (TEA) ESD, a new traction system for improving submucosal cutting line visualization. DESIGN: Case series. SETTING: Okayama University Hospital. MAIN OUTCOME MEASUREMENTS: Efficacy and safety of the TEA-ESD procedure. RESULTS: Three cases of large, flat, elevated colorectal tumors (laterally spreading tumors) in the rectum and rectosigmoid colon were safely and successfully removed en bloc without complications. Total procedure times were 3 hours, 40 minutes, and 30 minutes with resected specimens measuring 70 x 68 mm, 38 x 35 mm, and 30 x 20 mm, respectively. LIMITATIONS: TEA-ESD was performed in only the rectum and rectosigmoid colon. CONCLUSIONS: This limited case series demonstrated that large laterally spreading tumors in the rectum and rectosigmoid colon could be safely resected en bloc with TEA-ESD.