Background: EMR of large sessile polyps and laterally spreading tumors (LSTs) of the colon is safe and cost-effective. Perforation remains a feared and well-recognized complication; however, endoscopic detection is often absent, and most commonly, diagnosis is delayed and depends on clinical signs and/or radiology findings. To date, an endoscopic sign to identify muscularis propria (MP) resection and potential perforation has not been described. Objective To describe an endoscopic sign for prompt recognition of EMR-related MP resection. Design Prospective analysis. Settings Tertiary referral academic gastroenterology unit. Patients Patients with the target sign were identified prospectively in 2 large prospective studies of EMR for colonic LSTs 20 mm or larger. Intervention A standardized EMR approach was used. MP defects were closed endoscopically with clips. Main Outcome Measurements The presence or absence of the target sign in the polypectomy specimen and its influence on subsequent endoscopic management of polypectomy complications. Results A total of 445 patients with LSTs or sessile polyps 20 mm or larger (mean size 33 mm, range 20-85 mm) were prospectively enrolled in 2 studies. Ten patients (mean age 70.3 years, range 48-83 years, 50% male) with target lesions and histologically confirmed MP resection were identified prospectively at the time of EMR, with 3 having full-thickness resection. All cases were identified intraprocedurally by a target sign on the underside of the specimen and a mirror target evident in the mucosal defect. All patients were treated endoscopically with 1 to 5 endoscopic clips. None required operative management. Thirteen inpatient days were required to treat the 10 patients (mean 1.3 days). Limitations Nonrandomized study. Conclusions Careful analysis of the post-EMR specimen and resection defect may reveal a target sign, an easily recognized and reliable marker of either partial- or full-thickness MP resection and potential perforation. Prompt recognition of this sign facilitates endoscopic management.